Sometimes you do. We've seen a dramatic amount of "overfunding" of social media startups, and I have a theory why.
When your business model doesn't involve well, actually making money at its core, you end up in a position where you have to staff up massively so you can pursue an enormously vast range of monetization strategies in the hopes of finding something that sticks.
How many people on Facebook do you think are working on the core product - as in the Facebook that end users interact with on a daily basis? How many instead are working on a enormously numerous systems that Facebook hopes would make them monetizable? Ad platforms, billing systems, support systems, analytics systems, etc etc.
So yeah, Twitter as defined as "that thing where you submit strings of 140 chars or less and follow others doing the same" probably didn't need $1bn to get off the ground. Twitter in the "this might actually make money" sense though, probably isn't THAT overfunded.