There’s no question that search engines can be a tremendous source of traffic. Social networks are also proving to be big traffic generators for a lot of content producers, and Twitter is one of the big ones.

However, it is Facebook and Digg that are driving the most repeat readers according to a study conducted by online ad network Chitika. Traffic is great, but traffic that returns is even better.

The study was based on 33 million unique users across Chitika’s publisher network in September. It compared the number of visitors coming from major traffic sources Digg, Facebook, Google, Yahoo, Bing, and Twitter, to the number of times those visitors came back to the referred site. Visitors that went to a site four or more times in one week were considered loyal users.

By a wide margin, Facebook led the pack in providing loyal traffic. 20% of all visitors from Facebook visited the site four or more times per week. Digg was second with slightly over 16%. Digg was followed by search engines: Yahoo, Google, then Bing with 15.89%, 11.84%, and 11.74% respectively. Finally, Twitter came in at 11.08%.

"While Google naturally wins in sheer numbers, if a website owner can drive a thousand people to their site via Google, or they can drive the same number via Facebook, Facebook seems to be the way to go for creating a loyal reader," says Chitika.

The sheer popularity of Facebook and the time spent on the site must play crucial roles in driving repeat traffic. When a user frequently checks for status updates, they are likely to see new posts made from Facebook Pages they are subscribed to. This shows that creating a Facebook page can have a great effect on content publishers.

Twitter has historically been harder for users to manage and keep up with. That may change, however, once the Lists feature reaches the masses.