This isn't the first time Twitter has bought a pre-launch company for this much, and the answer is probably the same way that Twitter managed to value Vine at $30 million before its launch: http://allthingsd.com/20121009/twitter-buys-vine-a-video-cli...
For any pre-launch company, this would mean looking at the product, the team involved (as in acquihires), and the fit with the parent company.
For any pre-launch company, this would mean looking at the product, the team involved (as in acquihires), and the fit with the parent company.