It's definitely challenging.
One part is actually meeting people with whom you click. Unless you go to a lot of meetups and similar events, overall, you won't be meeting that many new people besides co-workers. And meetups tend to have a theme other than just having fun, whereas that's what most college activities are about. So you meet people in a serious context, and you need to bring the fun with you.*
So, you first need to come across someone with whom you click, and then you have to fuel that relationship. What's difficult there is that you need time. Once married with children, grabbing a beer after work is not as simple. You also have that awkwardness as pictured in "I love you, man" (and I believe a Louis CK sketch in his TV show): you have to acknowledge pretty fast that there's something going on and that you feel you could indeed become friends! It's one meetup, you're awkward now with that guy who seems cool, or you might never see him again.
But as you grow older, you also lose a lot of that casualness that you had before: you don't just invite them over after work to play video games. Now that you're a "grown-up", it's "a dinner". And since you're a grown-up, your home has better be somewhat clean, and you can't just serve pizza. What was a spontaneous interaction before is now a whole event that needs planning, some cleaning, some cooking, etc. (until of course, you become real friends) So these happen less, and friendships can be like making mayonnaise: if you don't make it go right now, you'll never make it go at all.
* I'm not saying meetups are no fun, just that it's not the end-game for everyone. Some people are there only for "professional" reasons and you need to figure out what people are looking for.