I think Hackernews has 220000 registered users. You are one of them.

You have 17 bits left. Use them wisely.

This is fallicious. Anyone can register for an account. Knowing someone is on HN only gives enough information, that said person is in the 'HN demographic'. Just because he happened to register for an account, vs someone similar who didn't, does not give us the amount of entropy removal you implied.

Since the GGP is easily googleable he actually has 0 bits left, but you are correct about the math.