After months of rumors that it had closed a second round of venture capital funding, Twitter announced yesterday that it had raised the money from Spark Capital and the VC firm controlled by Amazon founder Jeff Bezos. The size of the round was not disclosed, but a report by Om Malik pegs it at $15 million, with a $80 million valuation.

The funding seems timely. Twitter has been growing like gangbusters this year. Since my story on Twitter co-founder Evan Williams appeared in Inc.'s March issue, the number of users has more than doubled and traffic to the company's website has exploded.