Twitter and Facebook may be all the rage, but ordinary email is still king when it comes to sharing content online, according to new research from ShareThis.

The company’s “ShareThis” button has become ubiquitous at the end of articles and blog posts, linking to a widget that lets users share material via email, instant messenger and social networking tools including Facebook, Twitter, StumbleUpon and Yahoo Buzz. With user data from 200 million monthly visitors across 130,000 sites, the service has amassed a wealth of data about how people share and engage with content.

Email remains the tool of choice for that purpose, accounting for 46% of content-sharing activity compared to 33% for Facebook, 14.5% via other channels and just 6% for Twitter. People who receive links through ShareThis also tend to spend time with the content, at an average 2.95 page views per click, followed by Facebook (2.76), with Twitter trailing at 1.66.

Where the microblogging service shone was in getting recipients of forwarded articles to click on links. Twitter generates 40% of the clicks generated by shared content, with email and other social properties accounting for 35% and nearly a quarter coming from Facebook.

 For every post sent to Twitter, 18 people will click on that link compared to three for Facebook and one for email.

“Twitter is a very effective channel for pure reach purposes,” said ShareThis CEO Tim Schigel in an interview. “But as far as engagement, it’s at the low end. That speaks to the fact that on Twitter you don’t know what a link is and because Twitter is all about the buzz, not the content.”

ShareThis, which has received $21 million in venture capital to date, wants to use the sharing data it collects to help publishers better monetize their sites and marketers hone in on “influencers” playing a key role in spreading content. The company estimates, for instance, that content-sharing is driving the equivalent of 15% to 30% of the traffic that search does.

“We think sharing is going to be driving more and more traffic, and potentially more valuable traffic,” said Schigel, pointing out that most users say they share material they think will be helpful. So if someone knows a friend is in the market for a car, they could send her a link to an article about hybrid cars. (Of course, they could also forward a link to a story about the latest revelation in the Tiger Woods scandal.)

But Schigel said publishers have limited knowledge of how their content moves around the Web. “If you ask a site manager, they’ll know how much traffic they get from search. But when you ask about traffic from sharing activity, they can’t tell you,” he said.

To provide greater insight, the company plans to introduce analytics and ad tools for publishers and advertisers to take better advantage of sharing-generated traffic. “Socially contributed traffic is going to grow and marketers will start to optimize it,” said Schigel.

In a telling sign of the rise of social media, comScore reported Tuesday that Facebook had crossed 100 million unique monthly visitors in November, displacing AOL as the fourth-largest U.S. Web property.