Even before dating-via-Facebook, etc., I think things were different.

From watching Happy Days, etc., I conclude that back in the day, people used to "date" multiple people in parallel, and were only monogamous if they were "going steady".

But by the late 70s to early 80s (when I was of the age), this wouldn't have been tolerated. You can only be "dating" one person at a time.

So maybe the doing-things-as-a-group thing is a work-around that's allowing a person to interact with more potential partners than the one-at-a-time rules would allow.