I'm not sure if they stay or not, but if they like your site well enough, they might give you their email address, and allow you to email updates regarding changes to your site.

I'm not saying it's a sure-fire way to get permanent users. I'm just saying that it's a cheap way to get a couple thousand users to hit your site. Whether they stay or not... Doesn't that depend on if they like what they see, not on how they got there in the first place?