My friend Pete Kaminski suggests (on a private Facebook forum) that there are two reasons that Facebook has taken a big chunk of our online social time – "ubiquity", which is the character of providing global communications at scale, and "focus", which is the ability to narrow your personal interest discussions down to a small set of topics. For all of its other charms, Facebook provides both of those, imperfectly of course but well enough that it can serve as a does-everything social platform for lots of people.
In addition to ubiquity and focus, I'd like to suggest a third axis of utility for social media – "isolation". It's the notion that you can go somewhere online and do something special that's unlike anything else on the net, and that because of this the rules that have accreted over time about expected behavior are different.
As an example, I have an account on Pinterest that I use nearly exclusively for collecting maps. I don't follow all of my friends there, and I don't expect that people will follow me just because they are my friends. Similarly, a few friends have Plurk accounts in a little circle where no one expects that anyone else will read what they write and where there is a decided element of silliness that would not play on Facebook.
I think we all need a little corner of the net to call our own. Facebook ranks very low on the isolation scale and so it will not appeal to those who are looking for a quiet corner to express themselves. A lightly trafficked wiki, your own blog, or the carefully walled off corner of an interest-specific forum can all have that isolation factor that lets us be sociable but not to the entire world all at once.