If you are a Hero member, then you can get as much as BTC0.0014 per post. That is equivalent to $0.63 per post. This rate is much higher than what you'll get for normal forum posting jobs ($0.15 to $0.20).
Those signature campaign mostly need constructive post. It can be hard to get constructive posts. Still, a good way for forum user to earn some small amount of bitcoins through posting in the forum.