Check-ins were an evolution of the status update, but they lack mainstream appeal, have privacy related issues, and are limited in scope (you have to move to check-in to a different location).

Ok, I actually LOL’d at this one. Checkins lack mainstream appeal because they force people to actually push away from their computers and move to a different location? That’s kind of the point. 

Oh, and to answer this dude’s question as to why Q&A services are the hotness a) SEO and 2) SEO.