Meetups aren't as effective as you think. I have seen complaints from multiple large European cities that general social meetups don't actually result much in people exchanging contacts and meeting again in future; it feels like a one-off encounter with other human beings, and that makes it dismal. There is even a predatory industry now of for-profit social meetups because people are so lonely, and knowing that one is being exploited only makes it more dismal.

For more specialized hobby meetups, it is a common complaint that one's fellows are only met while practicing the hobby, they never become general friends that one can meet regularly with, let alone turn to in emotionally heavy times.

I wouldn't recommend solo travel as a way to socialize unless one knows the person's age. It's a sad fact that over the age of about 30-35, solo travelers (especially men) can no longer take advantage of the social opportunities of the backpacker scene; you might get a few nice chats, but most travelers will be very wary of you.