According to a report published this week by Shareaholic, social image-sharing site, Pinterest, has driven more referral traffic in the month of February, 2011 than Twitter did, and it is quickly gaining on Bing for referral traffic.

Shareaholic aggregated data from more than 200,000 publishers to compile the report.

There are a couple of points of interest to these numbers. First of all, Twitter has more than 100 million active registered users. Pinterest has only 11.7 million. And, it is worth noting that Pinterest is still invite-only and that their mobile app still stinks (and yes, I have deleted it and reinstalled; yes, I do have the latest iOS).

So, in all the guessing about why Pinterest is doing so well, let’s toss out a couple of theories.

1) It’s the design concept.

That certainly seems to be what some people are thinking. The “masonry” layout has been picked up by several other sites, all pining their hopes on catching the Pinterest wave.

2) It’s caught a strong, heretofore-underserved niche in the women’s market.

It certainly is true that Pinterest gets a huge chunk of its business from women. And, the powers-that-be at Pinterest aren’t doing a thing to dissuade that notion. Why would they?

3) It’s rolling like a snowball because people like me keep pushing it.

It is the media darling, that’s for sure. By all rights, an invite-only, badly-apped, female-centric site would not normally do as well as this is.

Maybe it’s all of the above. If Pinterest survives the copyright controversy blowback they are getting, they could make some serious money. How they would do that is till anybody’s guess. Apparently, including theirs.