Where is innovation happening now?

Currently reading Walter Isaacson’s “The Innovators” and I’m noticing the importance of locales: Bell Labs, Xerox PARC, Bletchley Park, et al.

Where are smart people hanging out and conversing and congregating today? I’m particularly interested in connecting online (HN is great!) and collaborating (or at least discussing) on the latest technology. I’ve been on Twitter, but I’m still not sure how to find people / get involved?