i only have experience of ballroom socials. It varies a lot. I Believe it depends on location. At some locations, you clearly have couples going to dance. They mix-and-match to some extent, but they primarily dance with the person they came with, and unaccompanied dancers soon stop going to these locations, as it's difficult for them to find a partner. Other locations are known to be good for unaccompanied dancers, and there's usually a much better atmosphere at these dances.
The band has something to do with it too. It's not just the music- it's the personality of the 'front' man (or woman)that makes the diference.