They are VC funded to the tune of around $8 million for now, I believe. There is talk of them charging for "premium services" and custom domains/user handles. Not sure how much they plan to make through that, however. Seems a bit of a flimsy business model to me. But these tech companies don't care much for robust business models. It's always a case of building a user base then worrying about the money later (usually by monetising the user base)