I ctrl-F'd "critical mass" and it turns out the point has already been made!

I had a similar idea years ago (and never got around to doing anything) and concluded I'd do the same as you, focus heavily on getting enough people using it in one place at a time...

The other thing I'd say is that "event" could have a broader meaning than the meetup/facebook definition. Sale at a local shop? Someone's giving away a piano? Yoga on the beach every Tuesday? I'd be happy to get notifications of everything like that within walking distance of my house, and the best part is if you focus on a single city initially you can scrape & ingest all that stuff from existing big & local websites to make Radius immediately useful without waiting for event organisers to sign up.

That's what I would have done if I wasn't so lazy, anyhow.