Make new friends, but keep the old...

The song we used to sing as kids makes it seem like making friends is so easy. Maybe it was when we were kids having fun on the playground, but I haven't found it to be very easy as an adult.
After 'little girl' was born I had a tight-knit group of new mom friends that hung out at least once a week. We all met in a Moms support group at the hospital where we delivered, sharing tears and laughter about the trials of caring for a newborn. More recently I've met other moms at playgrounds and through organized groups, like our neighborhood Moms group. I've definitely enjoyed meeting new people and have made some good connections, but unfortunately developing a new friendship takes time.
I'm willing to wait it out, but please share suggestions on how to meet new friends as an adult. Are there any local bakeries or coffee shops around town where moms seem to linger and meet up?
Has anyone used the website www.meetup.com to make connections? Please share any good or bad experiences.