Facebook Dwarfs Competition in Terms of Referral Traffic
But not only is Facebook by far the most important source of social media traffic, it is also the fastest-growing one. In September 2014, Facebook referrals accounted for 22.4% of traffic to the sampled websites. That's an increase of more than 115% compared to September of last year, when Facebook drove 10.4% of traffic.
Twitter's role as a traffic source pales when compared to its larger rival Facebook. The share of traffic driven by Twitter referrals recently dropped to 0.9%, down from 1.2% in September 2013. The second largest source of social media traffic is Pinterest, which accounted for 5.5% of visits to the sampled websites in September.

Description
This chart shows the percentage of traffic to a large sample of websites that comes social media websites.