Mediocre numbers from what I've seen. Every post-prelim year research fellow I've met, failed to match into derm the first (and sometimes second) time. They are hoping to get articles and book chapters in to pad their CV.
Sometimes even this is not enough...one girl I know is in the middle of her research fellowship, and has failed for the third time to match, even though the program historically has taken ~99% of its fellows. Her own program rejected her.
Same for another *big name* program with 8+ derm research fellows $$$$$$...they did not take ANY of their fellows. I do not know the outcome for these fellows...and did not see their names on this years' match list. Instead they filled it with people not previously affiliated with their program (ie, not rotators or their own med students).
I am assuming that all of these candidates were weak. No doubt, dedication to derm research is a strong showing...but when the rest of the app is weak, this does not bode well for the applicant no matter how many publications/connections she has.
But there ARE many research fellows who match! Don't get discouraged. This is just my personal experience.