[W]ithin 3 blocks of FeeFighters office are: 37signals, SproutSocial, Threadless, Crowdspring, and several other startups. I don't see how it helps us significantly to bump into them ...

That's because the marginal utility FeeFighters will get on average out of any given serendipitous moment is a lot less than, say, an unfunded startup no one's ever heard of.

If you graphed that marginal utility over time, you'd probably find that it's multi-region: linear in some parts, exponential in others.