MIT Techtalks saturday - TED speakers, MacArthur fellows and more. (techfair.mit.edu)

I'm biased: some floors of this building I'm in have an MIT building number for mail delivery.

I'll SWAG roughly 5,000 people read HN and are within walking or T distance of MIT.

I would guess it's smaller than you think. I feel it's not that big of a subset of MIT CS majors who care about startups/the kind of things posted on HN. Including faculty, I'm going to guess 200-300 regulars at most and up to a 1000 with occasional lurkers.