So we've been running a big program in all 4 engines that offer PFI/TF, a couple thousand pages in each. Ink is the largest referrer by far. They stack up like this:
For every 1000 referrals from Ink we see:
Ask/Teoma - 80
FAST - 25
AV - 20
Obviously there will be differences depending on the market and keywords targeted but if you are making high level decisions as to where to put your efforts/dollars Ink is BY FAR the best.
This is especially true when you figure that LookSmart effectively blocks some of the top keywords these pages in Ink might show up for.
Ask has been climbing lately, AV and FAST are going nowhere in our stats and would seem to be waaaaaaay overpriced for anyone looking to join PFI programs.
Unless OV gets bought out or gives AV and FAST a serious push, things may not change much.
Go with Ink, consider Ask/Teoma, skip the other PFI programs.