There are several people I consider (very) close friends, that don't live in the same city as me anymore. We message regularly, do some online gaming here and there and visit each other as often as possible. And every time we do it's just like back when we hung out in uni every day. While we don't have as much time as back then, the quality of friendship is the same or even grew. I think it's about consciously making time and the effort for each other, even if it can't be every other day or week.

Also not having kids makes it much easier, time wise, I guess.