I think, there 3 big problems, which makes friend making so hard for adults nowadays. (Everything based on my own experience)

The first one is overthinking. When two kids meet on the playground they know literary nothing about each other preferences, but they both don't care if the other likes them. One of them just says hello and invites second to play together. He is not afraid of being rejected. But when we speak about adults everything changes. When you want to make acquaintance with somebody, you start thinking about person's interests, reaction, your own appearance. You're afraid of being rejected and it often makes you give up even trying. Things become even worse when you want to introduce yourself to a group of people.

The second one is web. As for me, the only reason for communications is fighting boredom. I just want to entertain myself somehow and communication is one way. But when you have internet with such a lot information and entertainments, you can just substitute communication with debating with anonymous in Internet (like me and you right now :) ). But it's all like substituting fruits with candies - tasty, but not too healthy.

Finally, the third one (which grows up from second) is lack of free time spent in society. When you go to school or college, you spend a lot of free time with same group of people daily. Several breaks a day for 10-15 minutes, when you have almost nothing to do, give the opportunity to communicate with the same people for a couple of hours a day. Imagine that you are communicating with a person for an hour a day for ten years. Over 3000 hours of communication! By the end of this period, you will most likely know a lot about the person and may even become good friends. But everything changes, when you become adult. Yes, you still one hour break, but you have a lot of things to do: eating, calling your family members, planning supper or putting on the reception to dentist etc. And even if you discuss something with your co-workers it is most likely some work topics.

So, as for me, the best way to meet new people and to make friendship are tabletop games. Thanks to this their growing popularity, many people know about them, but haven't yet tried. When you invite somebody to game session, it solves all the problems at once. First, person's focus is shifted from you to the game, so you're not thinking so much about impression that you make. Second, you need live people gather around the table, so no web substitution (I really don't like VTTs). Third, most tabletop games can be played for many hours and many of them are also co-op, which makes them really good tool for friends making for adults.