My chapter was quite small, so we made it mandatory to attend 50% of social events, barring illness - but we counted semiformals and formals as two events. We always had trouble getting people to mixers, especially if they had boyfriends or fiances who weren't in the fraternity in question... giving formals more weight was a concession to this group. (I did have to give my GDI fiance the "yes, I'm going to a mixer, no, I'm not going to hook up with some frat [sic] boy" speech more than once
)
Events involving alcohol could not be mandatory, but I do remember one mixer where we sat around, chatted, did our icebreaker activities, then our social chair got up and loudly announced "The formal part of the mixer is now over, you can stay or not as you choose" and out came the beer.