sb9 I'm not sure what I've done about friendships, really. I'm completely hopeless at small talk, so find the getting-to-know-someone-new bit very difficult. Always have, probably always will.
I had a year off work and made an attempt at going to mother&baby groups, but tbh I didn't really click with anyone. I was a good 10 years older than most of them, had nothing in common with them and simply didn't fit in. I found it's not quite enough just to have babies in common. It was a relief to go back to work and have DD in nursery - she could get on with making friends without me holding her back.
Some of my pre-baby friends haven't bothered with me since I had her so I just let them go. Some other older friends are still around but I don't tend to see them because they are off doing other things.
DD is now at school and I find the parents (ok, mothers) very clique-y. It's like being back at school myself! I have made a good friend of one of the mothers, though, and we go out with and without the kids, but I think we could have become friends even without the kids, they just helped us meet. Others I pass the time of day with, but nothing more. I have made some attempts at getting playdates sorted out, but not very successfully (still don't fit in!). One or two I wouldn't wee on if they were on fire Meanwhile, DD is fine making her own way and I'm sure things will get even easier when she reaches an age where I'm comfortable letting her go to call on her friends on her own.
I used to look at other people seemingly making lots of friends effortlessly and envied them with an absolute passion. I'm sure none of it was really effortless, it's just the way it can look when you're struggling yourself.
I can't see any constructive advice in what I've just written, you know! But if it helps at all, I can completely empathise with what you are going through.