How many people used AIM at its peak? How many use it now vs. Skype / GTalk / etc.? Watching the shifting sands of Friendster / Orkut / MySpace / Facebook has made me believe that Facebook's hold on its currently vast user-base will always be tenuous. It will never be entirely irrelevant (same can be said for AOL) but has no guarantee that it will continue to dominate.
One social circle at a time.
Ask MySpace and Friendster.
one foot in each
I use facebook and Orkut. Sure, I use one more than the other, but there's possibility for that to shift because I'm free to use both.