This chart shows Facebook's referral traffic as a percentage of publishers' total traffic. In January 2014, 16.2% of the sampled publishers' traffic came from Facebook.
As a note: The numbers show the share of overall traffic on websites from publishers who reach 250 million unique visitors per month. Next to social referrals, that traffic includes direct traffic, organic search, paid search, etc.
Follow up:According to Shareaholic, Facebook referrals accounted for 16.2% of publishers’ overall traffic in January 2013, up from just 10.4% four months earlier in September. This is mainly due to a steep increase in referrals from Facebook’s mobile website/app.
You will find more statistics at Statista