This is the natural cycle of the internet, sites come and go. Look at myspace, yahoo, cycleforums, sites come and go. Major changes take place that people dont like, so they start migrating somewhere else.
Its gonna take a while because people have invested a lot of their lives on facebook, pictures, friends etc... its gonna take a while for folks to move to something else that is free and facebook will be just like yahoo, still out there making money (somehow) but nobody ever really uses it.
My guess is that as time moves on G+ which has sort of floundered for a while and been the other less popular social network will gain customers and start to get a decent foothold with more active users, especially once it becomes easy to migrate photo albums and friends list over there.
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