Now that the internets have expanded exponentially with crap and people gaming the search process, they have dropped down to Good Enough.
Most of us live compartmental lives. We're constantly separating various aspects of our lives, cultures and relationships. People, I think, form pretty cool organic amalgamations like large cities, universities, internets, etc. But planned spaces, such as malls, airports and suburbs, tend to be insipid. Facebook is the insipid mixture, IMHO.
I don't know if all social networking sites are equally doomed, but I think it's about the next thing and not necessarily the "next best thing."
Since people don't use friend lists, I believe Facebook will one day offer an auto-compartmentalize feature where it will intelligently build groups for you like talked about here:
http://www.slideshare.net/padday/the-real-life-social-networ...
Just as Facebook used the collegiate social scene to grow, I think theres a lot of potential for a competitor to take a similar route. Find a relatively self-contained community and attack it. It wouldn't surprise me in the least if a Facebook killer eventually emerges from the university system just as Facebook did.