> Do you think the shift away from these sites will be structural/technological (again, like if the world generally moves towards a different business model than data collection), or do you think these sites will be hit for specific reasons?

No. I think it will be completely cultural. In other word, exactly what happened with MySpace, AIM, etc. Young people will want something new, different, novel. The underlying technology and functionality will, in all likelihood, be exactly the same.