I don't have hard numbers either, but I think it depends a lot on where you do residency. If you come from a top program, you'll have the advantage of your program's reputation in addition to the fact that these programs usually have their own fellowships which fill internally. For example, in MA, the match rate for Anesthesia residents from MGH, BWH, and BIDMC is pretty much 100% and they pretty much stay at their own program or one of the other two. This is definitely not the case for Anesthesia residents in the other programs here. This is due in part to the fact that there is only one other Anesthesia Pain fellowship and this is at St. Elizabeth's (Baystate reportedly did not take fellows this year). From what I've seen and heard, there are also more Anesthesia residents applying now. In the past, completing a fellowship wasn't vital for many subspecialties (Peds, Pain, Cardiac, etc.), but that's changed now and I think this will make getting a spot more difficult in the future.
I disagree with the notion that Pain fellowships aren't competitive because they are routinely filling with residents from other specialties. Several of the top Anesthesia Pain fellowships routinely take PM&R residents because they recognize the importance of multidiscplinary training for their Anesthesia residents and fellows. These programs could easily fill with Anesthesia residents only.
I am definitely not saying that a Pain fellowship is extremely competitive, but if you are in a mid- to lower-tier Anesthesia residency without its own Pain fellowship (of which there are quite a few), I doubt the match rate is well above 90%.