Keep in mind, if you want to go on one of the over-populated servers, there are some drawbacks, like the fact that you won't be able to buy a house. Open world content (especially hunts) may be much harder to take part in. And you'll tend to get more lag issues there. The only real advantage to counter all of that is more people using Party Finder, but Duty Finder is cross-server anyway, so unaffected by how populated your server is. If you simply want a more socially active game, that has more to do with which FC and linkshells you join, and I'm pretty sure there are both active and quiet ones on every server.