Just my guess, but I bet that from this point on, few people (in the developed world) will race to sign up for a new social network. Why? Given the societal and personal costs (and benefits) that we've seen with previous social networks, a lot of lay people are re-evaluating the utility that such services provide them. In many cases, the answer is to keep your current social network, but increasingly disengage from it. Sometimes people find that they actually prefer to be less frequently "connected" to people in their lives, whether family/friends or casual acquaintances, especially given some of the caustic personality traits often displayed on social networks.
Also, many of today's recent non-FB social network successes (Whatsapp, etc) were launched before the general perception of social networking, and internet services in general, became increasingly skeptical. Outside of tech, or people with very narrow interest verticals not served by mainstream social networks, I don't know anyone who is looking for yet another generic social network.
Curiously, if true, this plays both the the detriment and benefit of established social networks: their primary, most profitable users are not likely to flee, but they are also likely to be less engaged.