1. Kids rebel against their parents and don’t want to use the same things their parents use. Ironically this issue will be exacerbated by Facebook’s desire to force people to make everything public (“I’m grounded because my Mom saw those pictures you took at the party last night”).
2. The young are still building their social graph and can be more open to new things. So all those things that tie people to Facebook won’t be relevant to kids growing up today.
So new generations come into the market both inclined and able to pick new movements and hence unseat the existing ones.
Right now Facebook looks invincible because they have no real competition. They did a very smart thing in creating an API. In doing so they’ve tricked startups into making Facebook stronger with apps rather than creating competing solutions. So all Facebook has to do now is fend off the pathetic attempts at competition from large companies who aren’t agile enough to compete. So they have no real competition and kids have no real alternative.
But if an industrious startup with innovative ideas comes to market targeting the youth demo I think they could kill Facebook pretty quickly among that group and become dominant as that group grows up.