I've found Meetup alright for finding things to do with other people, but it doesn't work well (at least for me) to find people to be with. Meetup helped introduce me to a once-a-month board game group which is nice, but the problem is that the people I meet through this are sort of stuck in that "role": they aren't my friends and won't be, they are just people I play board games with. Whereas my true friends are people I've hanged out with, speak with weekly, confided in, etc., you can't expect to have that friendship intimacy from strangers who just want a regular activity.

My feeling is that we're losing the facilities to make general friends, and that void is being filled with the non-optimal but best-you-can-do interest-based groups. This is even more effective on the internet where you can find even the most niche group that may not have been possible when we were limited to our local area.

In short: Meetup is good for activities, but it doesn't fulfill the third space OP mentioned.

(This is my experience, YMMV)