By at least one measure, YouTube, Google+, LinkedIn, and Reddit are all dramatically declining in value. According to new research, each site saw its share of Web traffic referrals nosedive over the past year. In June, “share of visits” -- a percentage of overall traffic, including direct traffic, social referrals, organic search, and paid search, that sites receive -- for YouTube, Google+ and LinkedIn were 0.06%, 0.06% and 0.02%, respectively. That’s according to a new report from content discovery and sharing startup Shareaholic, which sources data from a network of roughly 300,000 opt-in Web sites with what it claims is a combined reach of more than 400,000,000 consumers. In particular, YouTube and LinkedIn each saw its share plunge over the past year -- shrinking 82.66% (0.29 percentage points) and 77.43% (0.05 percentage points), respectively.Google+ fared a little better, losing 19.81% (0.02 percentage points) of its share of traffic year-over-year. On the bright side, YouTube, Google+ and LinkedIn offer the most engaged social media post-click referrals. Still, “a social network is only as good as the connections is fosters and supports,” according to Danny Wong, marketing manager at Shareaholic. What explains the referral failings of YouTube, Google+, LinkedIn, and Reddit? For one, for “the majority of marketers and publishers, YouTube is not a significant driver of traffic because not all businesses have the sufficient resources to develop high-quality videos their audiences want to see and share,” according to Wong. "Making good videos is prohibitively expensive.” That said, “among the few that have developed popular YouTube channels, it’s a traffic gold mine,” Wong notes. Google+ simply lacks the size or carry the same influence as competing social platforms, by Shareaholic’s measure. “Arriving late to the game, Google Plus failed to grow user adoption of the platform since most marketers and publishers still do not quite understand its value and are likely overwhelmed managing their already claimed and robust Facebook and Twitter accounts,” according to Wong. As for LinkedIn, most sites have yet to realize the power of authoring and syndicating content on LinkedIn, by Wong's measure. That, and the site has seen a remarkable decline in share-of-visits, year-over-year. Industrywide, however, social media is becoming a key driver of Web traffic. The eight largest sources of social referrals drove 31.07% of overall traffic sites received last month -- nearly double the 15.55% of traffic driven during the same period last year. In particular, Facebook is absolutely dominating the field. The social giant now drives nearly a quarter (23.39%) of overall traffic to sites, and appears to be just getting started. Over the last year, its “share of traffic” skyrocketed -- up 150.49% (14.05 percentage points) from 9.34% in June 2013. By increasing its share 10.09% (2.14 percentage points) from the first through the second quarters of the year, Shareaholic suggests that Facebook stole share from the seven remaining social networks, which lost a collective 1.97 percentage points.