I find this has also happened at both playgroups, nursery schools and primary schools.
To be honest, I just let them get on with it and don't get involved.
Although, saying that, I have become good friends with two ladies from my son's playgroup as my son is best friends with one of their sons.
Talk about a small world - I moved from my last house 3 years ago but am still in contact with my friends from the old road. We meet up every now and again. (Although, that road was also cliquey). It seemed we couldn't all be friends together, one would get the "hump" if you were seen socialising with one of the others.
Anyway, I have been out quite a few times for a girly night out with my two friends from nursery. Last Friday, one of my friends from my old road popped round for a coffee. She mentioned about popping round to my old house last Friday night (I have become friends with the couple who bought it) for a drink.
When the girl who bought my house phoned me up, I asked if she was at work. She said she was and worked at a local estate agents. When I asked the name, I mentioned that one of my friends from school also worked at the same estate agents. It turned out they are really good friends and even stayed the night together when they had their firm's christmas do. Talk about a small world. Whenever my friend from the nursery mentioned this girl's name who she worked with and what they got up, I never put two and two together. Goes to show you have to be careful what you say as you never know who knows who else!