"there are plenty of competitors they could make use of instead to replace particular single features or utilities of Facebook. "

" because dozens of comparable offerings exists at low or no cost that can take over subsets of the functionality in people's lives."

No, the value is in the network, not the 'feature'.

You can't go 'somewhere else' because 'nobody is there'.

This is not like Craigslist so much, the network externalizations for FB are everything.

The only way to break it would be to follow the same type of path FB did: Harvard -> Ivy League -> College -> USA -> World -> all ages.

A network could start say among some specific group wherein it was possible to achieve a critical mass, and move on from there.

So say you started where there was a specific need, like bands in LA trying to get the word out to gigs, among those that like to go to such things. And then it spread to other cities for music scene gigs, then for other kinds of entertainment, then more broadly into a social network. And you could call it 'myspace'. :)