If I may ask, what's the niche?
Otherwise, find related sites. If it is a niche that may have meetup groups, clubs, etc, then you could try to get them to take part/link to you by offering them a free membership.
I suppose you could also advertise on Plentyoffish, but it could be expensive and tough to pry their users away.
As usual, do:
Press Release
Offer some sort of contest for people that are join and are active
Encourage bloggers to blog about it - buy posts possibly