Absolutely crew socialise during layovers. If you go down to a bar (or a crew lounge) in any layover hotel, you inevitably find the crews mingling and generally hanging out with each other. Some crews have more of a "reputation" than others. That said, there are always a bunch of "slam-clicks" (named after the sound of their room door closing and latching right as soon as they reach the hotel) who prefer to stay home and watch TV instead.

As for "mingling" between pilots and FAs, well, boys will be boys and girls will be girls....