Ah, so that's how they're paying for the huge number of jobs they seem to be trying to fill all the time. Some days my entire job feed is just New Relic.
All three of these sites have a large amount of "promoted" jobs that show up. I'd recommend filtering them out with a script or user style.
I wasn't looking for a start up role, but have heard good things about AngelList[2] for those that are.