That's an argument that's in a way true, but it's also a non-sequitir comparison: in order to live in a middle class suburb (or especially a middle/upper class section of an urban area, e.g. Pacific Heights in San Francisco) there's a very high monetary barrier to entry. There's no monetary barrier to entry for Facebook (especially since it's no longer just for college students-- which may have been the seedling of t…
Which is why it's that much more disturbing that the separation is taking place anyways.
I can think of lots of situations where people self-segregate along economic or cultural lines, without much in the way of external pressure to do so. I'd imagine it has a lot to do with viewing oneself as part of a group, and wanting to spend time with people whose interests and background are similar, have similar values, etc.
I'd think this is something that academics have studied already; it shouldn't come as a great surprise that when people are released into some new environment they gravitate towards others similar to themselves.