LPT: Check on your friends who just had a baby. Ask if they need company, or anything, really. Keep calling them to social events, even if they are unable to attend for a good few months. Their routine has abruptly changed, hence most "friends" just vanish.

This is special if it is their first child and they are still learning how this whole new thing of how parenting works, if they live far from family and good safety net, etc.

Coming from a new dad who now feels left out...

Edit: whoa, this has really blown up! I didn't mean to start such a controversy. This isn't about the new kids, new lifestyle that will unfold. Most parents go through a storm and many important aspects of their lives are neglected, friendships bring one of them. My point is: reach out your OG friends, reassure, be near if they need, because they might suddenly be the weakest link for a little while.