Well, are we talking about using the group finder or just trying to get a group together on the fleet? Also, are we talking about low level flashpoints, HM flashpoints, or Operations?
From my personal experience leveling my Jugg and now my Shadow as a tank, until you get more abilities to hold the aggro beyond just Taunt, it can be a little frustrating not having the right tools for the job.
For HM Flashpoints, personally I only need to do 1/day. I'm not going to que in by myself if I can help it, since it will pretty much be an instant que, I'm going to take my friends. I would wager that most tanks think along the same line here.
For Operations, chances any half decent is in a guild and that is who they are going to run their operations with most of the time.
So in summary, beyond being one of the least appealing roles to play for most people in PvE, there are a lot other factors which make us more scarce than DPS.
Another piece of advice though, if you are DPS and if you want to stay off a tanks ignore lists, don't touch the mobs before we do.