You start out as strangers but I can say, after attending my local parkrun for a few times I got to know the other runners. Often it's half an hour of chatting at a nice tempo. Though I didn't meet any of them anywhere else, which isn't my intention anyways and I barely ever meet up with people.
You do the activity - you run and chat with some folks if you can. Otherwise, you just go to the social after. A lot of them meetup at a bar or something as the final destination to socialize post-run. Same with socializing before everyone is together and starts running.