If it was $50M on $500M post, then it's just a bit more than 10% dilution, which isn't too bad. But either way, that $50M buys two things: (1) a floor on near-term future price negotiations, and (2) a strong signal that they were nowhere near needing to sell and instead that they could go another 18+ months growing their walled garden of eyeballs. That second point could have spurred a big company like Facebook to overpay a little now rather than having to risk squelching an upcoming competitor a year from now at a GeoCities-like price.