They've only just started ramping their monetization engine as they've only been offering paid solutions for a few years now. They get all their revenue from essentially two products: promoted tweets and promoted accounts, so I see their revenue as pretty impressive given their simplistic offerings. They're now trying to get a piece of the $85B/year TV ad industry in the US too, so expect to see revenue start to come in from a variety of sources. For those criticizing Twitter because the majority of its users are a passive audience, remember that TV is in the same boat and commands the lionshare of ad spend globally. There's a lot of potential here, and they are just scratching the surface with what's possible.