I know people often recommend meetups, and I have attended a variety of those over the last year. The pattern I've noticed, especially on tech meetups, is that people show up in small groups with their existing friends or colleagues. If you come up and start a conversation, they mostly don't engage (bring up new topics, respond at length) - they just respond in a passive way to keep the minimum level of politeness, and wait until you realise they want to go back to talking with their friends. Starting a conversation with a stranger seems to be a rude thing to do, since you're making them uncomfortable.
The ones that do engage turn out to be non-tech (recruiters, marketers, people who want to start a tech career but don't know how), or people desperately looking for a job and thus checking if you can get them into anything.
What success I've had was:
- making friends at work (but for some reason it's always non-tech people - probably for the competition reason mentioned in the article - and it falls apart because they already have better friends they prioritise)
- starting a gaming group (but that had no followup - if I don't run a game, people don't invite me to their other things)
- making friends with a neighbour (an old lady open to friendship with anyone, that seems to have held up since she's retired and has lots of time)
At this point, I'm lost for ideas.
Google "London friends" or something, and you'll find people talking about how they're lonely and they'd like more friends. Seriously, where do those mythical lonely people exist?