Google apps are for working and getting things done; social apps are for interacting and having fun.
Facebook is a lobster trap and your friends are the bait. On social networks we are all lobsters, and
lobsters just wanna have fun.
A post from ifindkarma, to be read just for the lobster analogy.
Especially teaching is the Foursquare case: the founders previously built Dodgeball, sold it to Google (Google then shut it down in 2009) and then left – frustrated – to found Foursquare, which is based on the same idea.