Was 37 million around 2000 and now in 2012 it's 50 million. Another MMA payout article mentioned the UFC's fan base is around 8-10 million.
The writer pegs this on the influx of Mexicans but he doesn't really say why he thinks this. In fact he seems to contradict himself by saying another popular Mexican sport, pro-wrestling, has declined.
There are around 33 million Mexicans in the US and more than likely only a smaller percentage would be boxing fans. It's possible to attribute some of the growth to Mexicans but not most of it.
As far as boxing, which has also seen an increase in fans over the last 10 years, Dave Meltzer in the latest Wrestling Observer states (subscription required) that this is primarily down to the Hispanic market.
A big key is the emergence in numbers of the Hispanic audience, where boxing is the second biggest sport next to soccer. Those numbers have increased over the past decade from 37.4 million to 50.7 million, and will continue to increase over the next decade. What makes that strange is that pro wrestling is twice as popular with the Hispanic audience in the U.S. than the white audience, so whatever growth boxing should have because of it should be mirrored by wrestling. Itís hard to say why that isnít the case. Boxing hasnít had that superstar Hispanic since Oscar De La Hoya, and wrestling really hasnít had one in decades. Eddy Guerrero was close before his death and was the biggest since the heyday of Mil Mascaras (who was a superstar only regionally in the U.S.), but to really draw from that type of audience, the guy has to be No. 1, not No. 5 when it comes to being pushed as the serious top star of the brand.