I've never played mine craft but my observation came about because when doing paediatrics terms a lot of sick kids will have their parents iPad with mine craft.
I haven't seen the kind of deep social interaction that my brother sister and I had while playing lego though - more the way we all used to stand around while one of us played simcity 2000 (still social, but one driver of activity)
Two kids can share the same Minecraft play area in two different tablets/PCs. One runs a local server, which the other connects to. State is saved between build sessions.
I'm getting reports of a Bacon City and a Cake City being built connected by a railroad ;-)
I haven't seen the kind of deep social interaction that my brother sister and I had while playing lego though - more the way we all used to stand around while one of us played simcity 2000 (still social, but one driver of activity)