Doing research for someone, I found that Pinterest is now up there near Reddit.
More Info: Top 15 Most Popular Social Bookmarking Websites
Pinterest's global alexa ranking is already #53.
Alexa: Pinterest.com Site Info
Seeing as it's only been in the top 100,000 since the middle of 2010, that's outstanding.
We should definitely be keeping our eyes on this and start using this towards our advantage.