Products exist for this, but I'm not aware of any that have hit a home run. I think the biggest barrier is closing this gap: I personally want more friends since I don't have the social skills to reliably go proactively make a friend randomly out of a newfound acquaintance or friend of a friend. So I can go to a meetup, say, of people with similar interests. But I would need the aforementioned social skills - that I don't have - to convert those people into recurring "real" friends. Dating apps work better here because there's a much higher incentive for me to put myself forward in a way I'm not otherwise comfortable with. Vs "eh I have some friends already, I don't want to be awkward or embarrass myself."

I become increasingly convinced that it's not a problem that can be reliably directly intermediated for you. The best friendships I have that I was introduced to electronically came from recurring discussions around a shared interest on a site or forum or channel that then became a friendship. Trying to force things to go the other way is far harder. It either needs to be indirect OR you need to have an extremely high level of social skills (in which case you aren't likely to need this app in the first place).

Those recurring online discussions? That's social skill practice. That's putting in your reps. The Reddit or HN format is one of the harder ones for that; there are many better ones, though. But ultimately it all comes down to work and practice. In the same way that there isn't a pill or phone accessory that will build your muscles or teach you another language without putting in the work.