mrsnec - I agree that it has a lot to do with your personality and the things you enjoy, whether or not you will get bored/lonely/isolated. And whether you are in introvert or extravert!
When I moved to Australia 6 years ago, I didn't know anyone apart from DH and his mother, and DH was out at work, so I found that it was a real benefit to me to join a local playgroup or 2 with Ds1 once I'd settled into the house. I'm still going to both those playgroups now with Ds2 - and they've been the places where I've met most people whom I know. Now Ds1 is at school, I'm meeting more people there (parents and teachers) - but because I don't work I don't have other contacts. Dh has no friends, so that was a non-starter - and I think I would have gone slightly mad not having anyone else here to talk to. MN was one of my lifelines when I first got here! My link back to the UK and "normality" - gave me some continuity. Latterly, FB has become very helpful too.
Both my DSs are naturally sociable, will play with anyone, anywhere - but for a while there it looked like DS1 was going to be an only, so it was good to take him to places where he could meet and play with other children. Even better in that some of those children he met, he now goes to school with. One of the advantages of a smallish ruralish town in NSW, as opposed to the big city life! And the fact that schools here have the policy that if you're in catchment, you're entitled to a place; if they don't have space for you, they make it. So local children tend to end up in the same school.
It doesn't work for everyone - but the people I know, we talk about far more than just the children, even if that was our starting point.
I didn't go to playgroups with DS1 before we left the UK though, partly because I didn't need to (had my own things to do) and partly because I didn't feel that need to "network" on behalf of DS1, as we wouldn't be staying there. I took him to activities, but didn't really get chatting to other parents, except one who was at both swimming and gymbabes - and that was only because I held her DS once for her while she fetched the car, because it was pissing with rain!
re. 1st birthday parties - Ds1 had his cousins and my best friend's son at his. No one else. He didn't care, he had a lovely time running round after them! Then his 2nd birthday was in Australia before we knew anyone, so it was just a family day out to the zoo. Ditto 3rd birthday. 4th we went to the Harry Potter Exhibition in Sydney. He didn't have an actual party-with-friends-as-guests until his 5th birthday!
Each to their own though - you do what works best for you and your child(ren). :)