Seems to vary a lot by residency program and region. A lot of people in the northeast have started to avoid sports because of an over-saturated job market... while overall I think the specialty is still popular and draws many people to the field.
I would agree that peds seems to be a fairly uncommon fellowship, though some programs produce a number of pedipods.
The most rare fellowship by far is tumor, very few residencies provide sufficient onc training (due to its rarity) and there are very few fellowship spots nationally. Often you don't even see this specialty until PGY4, when most have already picked a specialty.