I think the most popular social network is Facebook. But as others said, people are looking for small services to solve a small problem.

studiVZ wasn't cool anymore. We missed the point for a rebrand to open the plattform for other people. "studi" is a abbreviation for student. Our answer was a new brand called meinVZ (just another UI, same database). Together with the ability to provide new cool features (e.g. Apps, Activity stream, i18n) it was just a question of time until FB was getting the market leadership.

Both questions can be answered with Facebook. For messaging though, Whatsapp is very common here these days. I don't like the service at all, but you're almost out of the loop if you avoid it - well, at least among my friends.