Eh. Maybe if the kiddos are close in age, not here though (my eldest is 11; my other two are 3 years old and 9 months old), lol.
Honestly hardly ever. They do have some social interaction but in the winter it was scarce. My oldest has boys scouts once a week and we go to the park maybe once or twice a week now that it is getting warm. We were in a homeschool group but that didnt turn out so great since the people were all over the place. We go to church but not as often as I would like. However despite my kids limited social interaction they are still very friendly and make friends every where they go. I think that siblings are sufficient play mates for kids and am not sure they really need outside interaction with peers. After all they get lots of interaction from going to stores and doing adult things with mommy and or daddy.
Weekly, for Sunday School/Church, dance class (before especially), and Thursday afternoon co-op for several hours. Monthly or so at a sleepover with her best friend. Sporadically at other activities, like birthday parties. Daily , she talks to her best friend on the phone or skype. In other words, PLENTY.