I've told this story before on eHell, but it's very relevant here. When the first of DH's friends had a child, it was a huge shock - to him. He couldn't believe how much his friends changed. Suddenly they couldn't go anywhere or do anything. Visits to their house revolved all around the baby. Their entire life, personality, and topics of conversation were all baby focused. It really affected him. Fortunately, a year later we moved away near to a relative of mine who DH made good friends with. They were just starting their family too, and had their first child shortly after we arrived. Yes, some things changed - as loving parents obviously caring for their child becomes a priority. But they still socialised, they still had interesting conversations, their house was still a welcoming, fun place to be at. (In fact, if anything they become MORE social at home because that way they had easy access to baby food and baby's bed etc so could slip off and sort baby out without disrupting the party). Both sets of parents are truly wonderful people, and are really great parents. The difference is that one set sacrificed their entire identity to become parents and gave up their own lives, while the others saw it as just another wonderful part of who they are and adapted accordingly. It was this second modelling of parenthood that encouraged DH - it's the type of Dad he wants to be, one who is a super Dad but still has his hobbies, friends, and life.
I think the problem is that new parents will always think that *their* way of doing things is the right way. It's hard for any feedback to come across as anything but criticism. I do think it's SS of Mary to be complaining so vocally about this - if people weren't accepting my invitations I'd realise pretty quickly it was something wrong with the way I was hosting!! It's sad that she's just driving people away instead. But I don't think I'd address it with her too directly as there's not really a nice way to say that you don't enjoy going to their house. All I'd do would be to encourage her to come out more. After all, she's invited. She's the one choosing not to go out and socialise. Life is choices!