I think the point is more that it's not that social networking sites aren't viable... it's that the web changes fast.
I think you are applying the lessons of the past a bit too liberally. For the last 15 years or so, "the web changes fast" has been quite good advice. But now it is maturing. The web has found a lot of roles that it fills quite well and, more importantly, has developed its own institutions. Amazon, Google, EBay, Slashdot (whatever their corporate overlords are called, I forget) - these companies have all been around for a while and they all have reason to keep innovating to stay on top.
Like it or not, parts of the web have become so ingrained into our lives that they have become more like utilities than luxury brands. Facebook is here to stay. It has critical mass and it offers really good features- and is working hard on offering features you don't even know you want yet. You see, that's the key. The internet's modern institutions are making money, and they certainly have the motivation and resources to create (or buy) the next best thing. Gone are the days when some guy in his garage could topple the current leader overnight.
The critical difference between Facebook and AOL? It's the same as the difference between Facebook and MySpace. Both thought they were content companies, but then competing content came out and they were made obselete (ie, AOL->Yahoo and the rest of the internet, MySpace->YouTube and then Facebook). Both missed the real opportunities in their niche- for AOL it was offering (broadband) internet access and for MySpace it was social networking.
The beauty of Facebook is that they don't pretend to be a content company. Rather, they are a utility, one that will integrate and improve the presentation and accessibility of any future content type. I'm not saying Facebook is invincible, because its income looks a bit small for the number of eyeballs it has and there's always the risk of mass user revolt over privacy concerns, but I'd give it a better than even chance.