ShareThis, which reaches 400 million people a month through social media share buttons across content all over the web, has released some new metrics for measuring social media effectiveness.

One of the metrics is its Audience Index, which lets publishers understand and compare their social audiences against 850,000 other sites (and soon against categories), find out what types of influencers your site attracts, and find out how well you connect with influencers, listeners, and engaged customers.

The other metric might be even more useful. That would be "Social Reach".

"We recently surveyed publishers (ours and others) and found that over 60% wanted (and were missing) social referral analytics," ShareThis explains on the company blog. "Social Reach measures the true value of shared media across the web by looking at outbound sharing and inbound social traffic and, in the process, gives proper credit to the listener/responder of a share as much as the original influencer/sharer. Publishers can now get a more accurate measurement of how a piece of content circulates around the Web after it’s been shared across any service, rather than just the simple number of shares counted by a single service like Facebook."

"Social Reach is built in to all of ShareThis' new share buttons that we introduced a couple of weeks ago, and the Social Reach score of each piece of content can be displayed on any page where our buttons are installed," the company says.

ShareThis shared some interesting stats regarding Social Reach with TechCrunch. For example, according to the company, Facebook accounts for 45 percent of all shared content across its network, followed by email with 34 percent, and Twitter with 12 percent.

While Facebook may get more shares from ShareThis, email appears to be more effective, as people are more likely to click on an emailed link (31 percent out of the 34 percent, compared with 36 percent out of FB's 44 percent).

Either way, the metrics could prove to be very useful for publishers looking at optimizing their social media marketing strategies.