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For unscheduled events, you would only want to know about people into the same stuff in your immediate vicinity. "Platonic Grindr"?

That would be fantastic, and definitely something I'd use. The idea of meeting up with a dynamically composed group of people to go and do an activity really appeals to me - especially with hobbies where new views/insights are particularly valuable.

Does something like this already exist, anyone (I'm googling and not yet finding anything)?

Other commenters have noted meetup.com seems to be the closest. I'm hoping something else more like this already exists, as meetup.com isn't exactly what I had in mind.

But please don't require people to have a Stasi 2.0/Facebook account to use your service.

meetup.com kind of does this.