I got curious and researched the history of IM, the decline of the old networks, and the rise of the new.

Anecdotally, I got into college (in the US) right as Facebook and Gmail opened up to everyone, and my peer group created a "professional presence" of Facebook and Gmail for our college lives, slowly leaving our myspace/AIM/WLM lives with the crazy hair and skinny jeans behind. My experience was not unique; this phenomenon has been researched by others [1][2][3][4]. Soon, Facebook and Google introduced chat, and nearly everyone I wanted to talk to was on one or both of those networks.

During my research, I was surprised to learn that AIM was in fact present at the iOS appstore's launch, but I suppose there wasn't much overlap between a typical AIM user and a typical iOS user at the time.

I also found an infographic from 2014 that charts some of these dates and compares the active user counts of the IM networks over the years [5].

Here's a chronological timeline of selected milestones in the IM/social space:

2005-09 - Meebo launches offering web access to AIM, WLM, Yahoo

2006-02-07 - Google Talk integration inside Gmail goes live

2006-03 - Nielsen/Netratings survey for active users: AIM 53M, WLM 27M, Yahoo 22M

2006-07-12 - seamless interop starts between Windows Live Messenger and Yahoo Messenger

2006-09 - Facebook opens up to everyone (not just colleges)

2007-07-07 - Gmail opens up to everyone (not just invite-only)

2007-05-09 - Windows Live Messenger released for Xbox 360 (with dashboard update)

2007-12-06 - Google Talk gets limited AIM interop

2008-04 - Facebook chat goes live

2008-04-19 - Facebook overtakes Myspace in Alexa ranking

2008-07-11 - iOS App Store launches, AIM for iOS released

2008-08-26 - Facebook hits 100 million active users

2008-09-23 - Android 1.0 launches

2008-11-11 - Google Talk introduces voice and video calling

2008-12-22 - Meebo integrates with Facebook chat and Myspace IM

2009-03-31 - Skype released for iOS, Skype network has 42 million active users

2009-04-08 - Facebook hits 200 million active users

2009-06 - iOS gets push notifications

2009-09-15 - Facebook hits 300 million active users

2009-11 - WhatsApp released for iOS

2010-01 - WhatsApp released for BlackBerry

2010-02-05 - Facebook hits 400 million active users

2010-05 - WhatsApp released for Symbian

2010-05-20 - Android gets push notifications

2010-06-21 - FaceTime released with iOS 4

2010-06-21 - Windows Live Messenger released for iOS

2010-07-21 - Facebook hits 500 million active users

2010-08 - WhatsApp released for Android

2010-09-30 - Windows Live Messenger starts interop with Facebook Chat

2010-10 - Kik released

2010-12-02 - Viber released for iOS

2011-01-05 - Facebook hits 600 million active users

2011-01 - WeChat released

2011-01 - Skype for iOS introduces video calling

2011-04 - Facebook introduces voice calling

2011-05 - Viber released for Android

2011-05-30 - Facebook hits 700 million active users

2011-06 - LINE released

2011-07 - Facebook introduces video calling

2011-07 - Snapchat released for iOS

2011-09-22 - Facebook hits 800 million active users

2011-10-12 - iMessage released with iOS 5

2011-10-13 - Microsoft finishes acquiring Skype

2012-04-24 - Facebook hits 900 million active users

2012-07-11 - Meebo is acquired by Google and shut down

2012-10-29 - Snapchat released for Android

While it's tempting to accuse AIM, MSN, and Yahoo for being incompetent and not catching up to the "mobile era", they in fact did pursue this market as much as they were able. In truth, early iOS and Android were inferior platforms for a chat app. Push notifications were absent, data rates were expensive, and the average smartphone user at this time was not very likely to use those networks anyway.

Based on this info, I reason that it was truly Facebook that killed incumbent IM networks, at least in the US. Between the release of the iOS App Store and the introduction of push notifications for Android, Facebook grew by more than 300 million active users. This coincided with exodus of users from Myspace to Facebook; many of those users likely having used AIM, MSN, or Yahoo messenger in the past, now found themselves in a much larger network that also offered chat. Since Facebook largely subsumed everyone a person knew in real life, these users only had to go back to the old IM networks to chat with people they didn't know in real life, setting the stage for the weakening of connections and these networks' decline.

By 2010, Facebook, or at least awareness of it, was mainstream. At the end of 2008, the Webster's New World Dictionary named "overshare" as the word of the year [6], while in 2009, the New Oxford American Dictionary chose "unfriend" [7]. For people new to the IM landscape, the old networks were dying and full of "old people" now in their 20s and 30s, so new networks surfacing around this time were appealing. This contributed to the grown of Kik and Snapchat, while people for cheaper alternatives to texting and voice calls drove the adoption of Viber, WhatsApp, and Skype. iMessage went live in late 2011, offering with FaceTime a built-in rich chat on iOS, successfully capturing an audience that would've surely gotten a third-party app otherwise. Later, Hangouts on Android emulated this strategy.

Skype is a remarkable special case. Microsoft managed to squander the popularity of MSN/WLM with its confusing product strategy, then it acquired a VOIP product that targeted a different customer base. Not content with running the two products separately, they deprecated WLM and encouraged everyone to migrate to Skype, which didn't happen. Later, they leveraged Skype as the built-in IM for their OS, while still committed to keeping it cross-platform. It could very well be a trojan-horse into the Microsoft ecosystem, but it's essentially entirely separate, completely unlike Google Hangouts.

So now we're living in a time when smartphones come with IM out of the box, nearly every social network is gaining IM functionality (Instagram, Twitter, Tumblr, etc.), and the new wave of circa-2011 chat apps are diversifying into social networks (Snapchat) or platforms themselves (Kik, Viber, WeChat, LINE). You actively use more than product capable of IM, but rarely by choice and mostly by acclimation. Ironically, this situation benefits platforms the more closed they are, an intuition that's made clear by complete decline of interoperability between platforms in the past. IM is ubiquitious, leaving old "IM only" networks owned by corporations who can't figure out what they're doing (AOL, Yahoo) utterly irrelevant.

Sources:

[1] 2012 http://socialtext.dukejournals.org/content/30/2_111/99.full....

[2] 2011-06-22 http://www.bloomberg.com/bw/magazine/content/11_27/b42350539...

[3] 2007-07-11 http://www.nbcnews.com/id/19717700/

[4] 2009-03-16 http://www.newsfactor.com/news/Facebook-Traffic-More-Than-Do...

[5] 2014-10-22 http://www.whoishostingthis.com/blog/2014/10/22/instant-mess...

[6] 2008-12-01 https://wordoftheyear.wordpress.com/tag/websters-new-world/

[7] 2009-11-16 http://blog.oup.com/2009/11/unfriend/