Sam helped him out and now he's repaying the favor. If it was $10M+ that's nothing when Zuck's a billionaire on paper. Additionally, it doesn't matter what the investors think as he still has complete control of the company. I don't think it's fair to compare Hot Potato and drop.io as there's a history with the founder that wasn't there before.
If I'm sitting on a runaway success, you better be damn sure that I'm going to help my genius friend (Sam's a crazy smart guy) out and bring him on whenever I can. He's only going to add value and when Facebook is expected to be worth $100 billion eventually (pure speculation right now but it's in that direction), there's nothing wrong with holding out a golden hand.
Nobody ever asked us to pay. We would have. But suddenly, bam! It's gone.
Was it a non-negotiable part of the acquisition? Or did he not even try to negotiate?
Seeing the recent development of the site and the evolution of Dropbox made it clear Drop.io wouldn't be around long term.
Drop.io obviously choked to death because it was too afraid that gesticulating its agony would be embarrassing.