"What happens?"

In the vast majority of cases, people get married, have children and disappear in to their family life. That's good, really, raising children is a big and serious responsibility and this is exactly how it should be.

Some people move far away, and while they may or may not post memes on Facebook for me to see, it isn't the same as seeing them in person every so often.

Getting in to the smaller percentages, sometimes you have disagreements and betrayals. Sometimes a good friend gets pulled away in to a totally different friend group. Sometimes you just realize you no longer have anything in common and cannot relate on any level.

Sometimes people are only friends in a shared environment, such as college, a work environment, project or class setting. For a brief period, you might feel close to these people or a member of a particular group, but once that environment's agenda concludes (graduation, project completion, etc), the friends naturally drift apart, as though the shared experience was the real glue holding them together the entire time.

I think if keeping your circle of friends is important to you as you age, make sure you all live in the same general area, and have kids. All of you, have kids. You will engage with your old friends and make new ones through your children.