It was expected that numbers might drop a bit when UFC programming switched from Spike to FX. The first prelims special that aired on the network, for UFC 142, played that out to a degree. But the sky isn't exactly falling. MMA Weekly has the numbers:
The UFC 142 Rio Prelims, which aired four bouts live on FX, drew just 880,000 viewers, which is the least watched UFC Prelims broadcast since the series began on Spike TV with UFC 103 in 2009.
That's kind of a half-truth though. Yes, the UFC 142 prelims did draw less viewers than any of the Spike episodes. But their article fails to mention the three prelims specials that aired on Ion. UFC 125, UFC 127 and UFC 140 all aired on that network, and all did worse ratings than this show (829k, 808k, and 714k respectively).
So what does this mean? Absolutely nothing, really. It just shows that a lot of people are still interested in watching the prelims, and that's not a bad first number at all in my eyes.