I run a guild with ~25 people in it. Not too big or small, and we all help each other when we need to run a FP or Heroics regardless of the level gap. We are well established, and have been for about 7 years, but we also have new people. Granted, finding the "right fit" can be a challenge sometimes, but a FP finder is a step away from community and socializing imo.
25 people is too big.
And no, FP finder is not a step away from community and socializing. All it eliminates is general channel spam of LFM or LFG. Which, despite protestations from some users on the forums, is not socializing nor does it build community. What you do WITHIN the group is socializing and builds community. This is not impeded in ANY WAY by a finder.