Not going to claim this is the authoritative reason for the G+ flop but as far as I can tell from the user perspective, it came down to a few related issues with the basic way social networks work.

Probably the easiest way to illustrate is to take the other examples you gave (search, mail, and browser) and compare to G+. If you're a user and you are used to Yahoo or Altavista but you hear about this new Google search, you can easily check out Google search and start comparing how it works relative to your usual experiences. If you like Google search better, you just start using Google search as your primary engine. Done.

If you had been user@yahoo.com for years and hear about Gmail, you open a Gmail account, start sending and receiving email as user@gmail.com and if you prefer the new service, you set up forwarding on your user@yahoo.com account and tell friends and family to use the Gmail address. Done.

If you had been using Safari and hear about Chrome, you download Chrome and use it for a few days. If you prefer your old Firefox setup you leave Chrome installed or ditch it but if you like Chrome, you just start using Chrome. Done.

But social networking isn't the same as email or a browser or a search engine. It's about communication like email but unlike email, you can't just pick whatever social networking service you prefer and use it. For social networking (at least in the typical Friendster/Myspace/Facebook way we usually think of it), you need everyone to be on the same service in order to communicate with them. Switching services is a lot more work than switching a browser or a search engine and there's no relatively easy "patch" like forwarding for email. For a massive amount of people and orgs, Facebook was the first big social network that they invested their time in. It was the first one that really got widespread adoption among people who may not really do much else on the web. It was the one that your mom and your boss and your neighborhood association and your local bartender all joined.

And when G+ came along, plenty of people (myself included) checked it out because we like checking out new services and Google has put out several that are at least competitive if not definitive (gmail, search, chrome, maps, etc). I won't speak for everyone but I found it much nicer to use than Facebook. There was no Farmville spam. Sorting the contacts that were displayed on your feed or who saw your posts was much more granular and visual than Facebook. The mobile app was much faster and more attractive. It was already tied to your email, voice/video chat, calendar, and photo/video hosting if you had a Gmail account. At the time, Facebook didn't have a lot of these features.

But regardless of whether you liked it or hated it compared to Facebook, it turned out to be irrelevant. Because unlike switching from Yahoo to Gmail or Firefox to Chrome, switching from Facebook to G+ wasn't something that only depended on your interest and relative acceptance of change or effort. Because unless all of your Facebook contacts did the same, it didn't matter which you liked more. The real limit of these kinds of services is that they're not interoperable. And if one (in this case, Facebook) gets a "critical mass" of users who might be resistant to change or switching services, it's nearly impossible to succeed as a competitor.

The only real way it could succeed is by offering something other than Facebook (which is what they're going for now I guess) but they will never be able to succeed as a better Facebook even if they build one.