I personally feel they should keep the sharethebus.com brand.

The opening of the TechCrunch article states "It’s actually a pretty great URL: Bus.com. It’s also a much better brand identity than Sharethebus ..." - however they don't say why it is a much better brand identity.

Simply being a 3-letter domain doesn't make for a better brand identity. They have traction and funding, so it they may not see much of an impact because it is still a utility... but how would you react to seeing a promotion relating to a festival talking about "Bus.com" or "Sharethebus.com" - which one gives you a feeling of something more than a utility?