Finally Twitter announces its funding. The popular microblogging/messaging service says its newest investors are Spark Capital and Jeff Bezos, via his Bezos Expeditions fund. That Spark was participating was already known, though Bezos’ participation is a surprise. Past backers Union Square Ventures and Tokyo-based Digital Garage also re-upped. The size of the round was not disclosed, though we and others have reported it at $15 million, with a sub-$100 million valuation. In total, the company’s funding comes to $20 million, as it raised about $5 million last year.

In a blog post, Twitter’s Biz Stone says the funding will address two of the company’s biggest question marks: reliability and the lack of a business model. Even today, the site has been overloaded and unusauble off and on, though its been awhile since a huge outage. So the money will go towards talent and infrastructure. As for the business model, it plans to find one eventually, but that’s still not the immediate issue. Says Stone: “Twitter will become a sustainable business supported by a revenue model. However, our biggest opportunities will be worth pursuing only when we achieve our vision of Twitter as a global communication utility.”

• Twitter Closing its $15 Million Round With Spark; Bit Less Than $100M Valuation