https://www.meetup.com/ is along these lines, but it's been around for years so it's obviously not a complete solution. There are special-interest groups on there but also broader "I'm in my 20s/30s and just want to be social" groups.
Thanks one example, but shows the overall idea: anybody going to a meetup event has already accepted that this a publicly known event where there will probably be some new people, and wants to socialize with new people too.