I'll take a stab at this one.

Google's main source of revenue comes from ads that are driven by user want. I search for "car insurance" and Google gives me an ad for GEICO. But that doesn't really tell me anything about GEICO other than that they have enough money to pay for my click. They could be excellent or they could be crap, but they do likely sell car insurance and I'm looking for car insurance.

On Facebook, I may ask my friends "what car insurance do you have and how do you like it?" I get better information; I get testimonials from people whose judgement I trust (or at least I trust that their judgement isn't paid for, unlike Google's ad). This cuts into the heart and soul of Google's revenue - matching advertisers with user needs. Facebook hasn't done this to my knowledge, but let's say they classify those queries and responses. So, I post the question and my friend Julie replies, "I have GEICO and I love them". Six months later, our mutual friend Jeff sees an ad in the sidebar with a GEICO logo and that quote from Julie. Better yet, Jeff posts a picture of his new car and an advertisement comment appears using Julie's quote.

This isn't about car insurance. Your friends will not be as good about filling search requests as Google for many needs. However, your friends might be better for the ones that pay. I have no idea if this graph is accurate, but it can be interesting to go through: http://www.wordstream.com/articles/most-expensive-keywords. Attorney and Lawyer are the #4 and $6 most expensive keywords. I think I'd rather go with a friend's recommendation than a Google ad for something important like that. We've already covered insurance at #1.

That's what makes Facebook a threat. Rather than searching, people ask the hive-mind on Facebook for a recommendation for a restaurant, insurance company, lawyer, etc. Friends and friends-of-friends comment and like different answers and while people are certainly fallible, at the very least is comes off as more trustworthy. If you were looking for wireless service in a new city, would a Google ad be the first place you looked or maybe how your friends in that city fared. Recommendations would certainly be skewed by the fact that they have too little data to have a good opinion combined with popular perceptions of the carriers, but would ad space that went to the highest bidder be more trustworthy?

Now, when I search for the Battle of Gettysburg, I don't get ads. Same for when I search for Washington DC. Same for many other search requests - the type of requests that don't really have money in them; the type of requests I use Google for and would never really use Facebook for. The threat is that Google would become the place you go for the searches that don't pay while Facebook gets the requests for information that do pay.

Worse, what if Facebook leveraged itself for videos as it did for photos? As the comment you reference mentions, Facebook's photo platform was worse, but its social aspect made it better for the way users wanted to use photos. Facebook's messaging platform makes it good to chat to a group of people who you know, but not super well - and everyone can see the whole conversation even if they were added later. Businesses are already setting up pages and giving Facebook nicely formatted data about the business.

The worst part of it is that Facebook is good at what they do. They're not some company that can't handle the technological challenges. While one can call their business fluff, their engineering staff isn't. So, the company isn't going to go away via technical problems.

I'm not saying that any of this will come to pass, but Facebook is certainly a threat to Google's revenue. Google makes its money off of ads that can be replaced by friend recommendations. Again, I'm not saying the replacement will happen, but I can see the threat.