In July of this year, online presence and social communications app Twitter announced that it had taken a Series B funding round from Union Square Ventures. At the time, USV wrote excitedly about the investment, even though they "don't yet know" what sort of business model can exist around the service.

One thing neither party disclosed was the size of the round. PE Hub reports today that a regulatory filing reveals that Twitter has raised $4.8 million of a $5.4 million round. Marc Andreessen, Dick Costolo, Ron Conway, Naval Ravikant, and Charles River Ventures are also investors in the site.

Perhaps no other small company has caused as much commotion as Twitter this year. Since it exploded on to the scene at SXSW last March (that's not when the service debuted, but when it finally found its audience), Twitter has been running away with headlines all year and their growth has been impressive.

Arguably one of the most important things Twitter has done is open up an API to outside developers (this is an argument that co-founder Biz Stone made to us in September). According to Twitter, the API has seen such incredible growth, that just months after it was released it is already getting 10 times the traffic of the Twitter web site.

One of Twitter's cheif rivals, Jaiku, was acquired by Google in October.