This is exactly what happened to Digg. You had people getting paid to get stuff up on Digg's front page. So Digg introduced friends and the ability to see what

This is exactly what happened to Digg. You had people getting paid to get stuff up on Digg's front page. So Digg introduced friends and the ability to see what friends are digging. And they banded together to really ruin the experience for others. During the time it happened, I did a bit of offhand calculation and found that of all the posts on Digg's front page, most had the same 100 or so "power diggers" in the first 200 likes of the submission. And these 100 users were authors of at least half the submissions on the front page, many of which were clear promotional ads.