Recently, a forum I frequent has received multiple spam posts on its general discussion board (three in the space of ten days). They all look like legitimate posts at first, but buried inside each of them was a spam link to an external website. I don't know whether it's the same website each time - I didn't check the first two before they were edited out by moderators and the threads locked.
Here's the odd thing, though: the link is always in 1pt text and coloured to blend in with the post's background, so it's completely invisible. You'd have to know it was there and actively hunt it down in order to click on it. So getting us to click the link is clearly not the goal here.
So what's the point? Why spam us with links that we don't even realise are there? There must be a goal here, but I'm at a loss as to what it might be.