As millions of Facebook and Twitter users know, the Internet, more than any other medium, is dominated by the speech of billions.

This has become a cliché but it’s true: The Internet is the greatest gift to free speech since the printing press.

Now, the Knight Foundation’s new study of high schoolers’ social media use confirms the role social media has played in this explosion of expression.

“This is the first generation in history that can text, tweet and blog to the whole world — it’s great news that their support is growing for the freedoms that let them do it,” Eric Newton, senior adviser to the president of the Knight Foundation, writes in the report.

The study says that high school students who actively use social media sites have a greater “appreciation of the First Amendment” than those who don’t. It turns out that people who  express themselves freely are far more likely to appreciate the right to free speech. The study notes that “fully 91 percent of students who use social networking daily to get news and information agree that ‘people should be allowed to express unpopular opinions.’ But only 77 percent of those who never use social networks to get news agree that unpopular opinions should be allowed.”

This means that privately run sites like Facebook — which the paper repeatedly refers to, given that it boasts more than 750 million users — have an overwhelming duty to protect the right of expression for their users. And judging from its continuing missteps on privacy and its willingness to arbitrarily shut down the pages of controversial groups, Facebook has left a lot of room for improvement.

That responsibility was made real earlier this year when Egyptian protesters, organized in part on Facebook, overturned the rule of Hosni Mubarak. Now, as the Occupy Wall Street movement builds and spreads to other cities across the country — in part because of its participants’ social-networking savvy  — Facebook, Twitter and other social sites will have to come to terms with their central role as platforms of speech, and find a way to balance their business interests with their users’ democratic impulses.