I try to organise playdates. About 40% of the time (based on a a small sample of 10-11 invites, admittedly) the child doesn't show at all, or the parent begs off last minute. Sometimes we get playdate invites back... the kids play nicely when they're here, that's not the problem. I try to invite 4 children at a time, now, so that DS doesn't get disappointed if some don't show, but I really can only do that on weekends or holiday times when DH is around to make sure they're all supervised -- if they do all come...
If only it were the cost! I bought some 5yo boy toys in the July sales thinking DS was starting a new school, would go to parties; oops! That was wasted. I'm flogging them on Ebay, now. I know that lots of parties and playdates are happening. The other mums chat about them. Once DH took DS to an indoor play centre where one of DS's classmates was having a birthday party... we didn't know about it. DH did know many of the other mums there, though.
There is a child (M.) who is in DS's colour group in class, they often eat lunch together & play together. Last week M. came to play (with 2 other boys, plus his big brother, later). They all seemed to have a good time; no fuss or bother at all. M.'s mum sat on the settee chatting to us for a while before he went home.
Today (yesterday now) I found out M. is having a birthday party this weekend. DS, of course, not invited...My neighbour's boy is, though.
I'll try to hope things change like you said, Marina. DS loves other children, party atmospheres. He is missing out so much.