Being a SENIOR FO is a choice, once you get to the point your seniority can hold captain.

As a regional guy I will normally buy a round of drinks, sometimes dinner. Of course we only have two FAs and we always end up at the same hotel. But as other said a lot of it depends on the crew. I’m not going to even go to dinner with someone I can barely tolerate the van ride with, and there are quite a few of those.