It's tough. There are two families that we socialize with pretty regularly at each others' homes, which is my favorite way to socialize. I credit my oldest son with introducing us to them, since he is friends with both older boys in those families. They were friends in montessori, and now all three families' kids go to different schools but we often visit each other homes on weekends or attend events together. I went to many many many many birthday parties and playdate attempts before we forged these friendships. Our oldest is now 8, and it took a while--since he was 4 or 5, I think. There have been others also, but they sadly moved away.
There is a family up the block, with kids the same ages as my older two kids, and I have tried to forge that link, but it just is a no-go. I don't like everybody, and not everyone likes me. The mom just doesn't really click with me, I think. Not sure why, but oh well. She is a SAHM, and I work full-time, and she used to make comments quite often about that. Not derogatory remarks, but still.
There's another family our age around the corner, and we all get along well, but we just don't seem to have the same schedule. They want to get together Friday evenings at the park after school, when we are still at work or want to unwind at home. We invite them for Saturday pool time at our house, and they're busy with sports or extended family or other things. We like each other, but it just hasn't worked. Partly, again, with that one, the mom and I just don't gel completely. I get along better with the dad/husband, which is not always great, I find. It seems to work better for us when the other mom and I are friends.
DH is way less social--he doesn't do birthday parties, he doesn't go to many events, he dislikes coordinating with other people; he prefers to tinker at home. This was an issue when we had no kids--most other couples wanted to "go out to dinner," which we didn't do. I feel grateful to the friends that we have for accepting and appreciating DH as he is.
Oh, and I have another group of mom friends who I met in prenatal yoga. We all had girls at the same time, and we spent our maternity leaves together walking around the zoo or the park or going to breastfeeding support group meetings. We've known each other now for about 7 years, and we do hang out on occasion, just not quite as often. And it's mostly the moms who meet; the dads are not friends (all hetero couples or single moms).