Some people did, but I don't know that it was a conscious "Oh, they have kids now so we can't be friends" kind of thing. Before we had children, most of our couple friends were of the "kids are great, but we don't want them ourselves" mindset, which was fine. They would call us at 4:00 on a Friday and we would meet them for drinks at 5:30. Fun!
Then we had kids... they would call us at 4:00 on Friday and we had to say "no". After a while, they stopped calling because we just couldn't do the same things anymore.
The upside to it is that we have since met new friends who have children. They understand that if they want to do something, we need some notice to get a sitter because they do too! They also tend to invite us over as a family and we do the same.
Friendships change as circumstances change. It's kind of sad, but it's also a part of growing as a family.