Answer a question about this article:
Google does not provide detailed figures for YouTube's running costs, and YouTube's revenues in 2007 were noted as "not material" in a regulatory filing. In June 2008, a Forbes magazine article projected the 2008 revenue at $200 million, noting progress in advertising sales. In January 2012, it was estimated that visitors to YouTube spent an average of 15 minutes a day on the site, in contrast to the four or five hours a day spent by a typical U.S. citizen watching television. In 2012, YouTube's revenue from its ads program was estimated at 3.7 billion. In 2013 it nearly doubled and estimated to hit 5.6 billion dollars according to eMarketer, others estimated 4.7 billion,
What was youtube's estimated ads revenue in 2013?
3.7 billion