The preliminary bouts for UFC 146 that aired on FX before the start of this past weekend's pay-per-view drew an average of 1.3 million viewers, according to MMA Junkie. That ranks fifth out of seven events to have been shown on FX (including UFC on FX main card broadcasts), tied with the UFC on FX 1 (1.3 million) and ahead of the UFC 142 prelim (880,000) ratings.
It is still 300,000 viewers under the UFC 145 prelim broadcast, despite 146 appearing to have the bigger names. This show had fan favorites Dan Hardy and Jason Miller as well as a former WEC champion Jamie Varner, which I thought would have driven ratings up a bit more. It's likely that the UFC 145 advertising presence, based around the strong push for Jon Jones vs. Rashad Evans probably had casual fans more "aware" that there was an event taking place.
For reference, the 27 prelim broadcasts that were held on Spike TV before the UFC's Fox deal did an average of 1.5 million viewers.