There is no reason why anyone should only socialise with the people they have been put in a flat with. Flatmates sometimes just end up as acquaintances not friends as they don't have much in common. I mean, don't ignore your flatmates but just chat if you see them in passing.
Definitely take the guy from the other flat up on his offer. As you spend more time with other people who like doing the same things you do, who you see at the events you like, you will probably end up with a bunch of people in other flats you hang out with - that's what happened for me anyway