When U.S. Rep. Anthony Weiner mistakenly sent the now notorious Twitter picture as a public message rather than a private one, he violated a bedrock tenet of smart social-media use.

When in doubt about the implications of the content, never push send.

"You're 100 percent more accountable for your actions online than you are face-to-face," said Jeremy Bailenson, founding director of Stanford University's Virtual Human Interaction Lab.

It's the kind of accountability that knows no end.

"Your digital footprint – the stuff you leave behind when you use social media – that stuff lasts forever," he said.

At Stanford, Bailenson studies digital footprints, and he's found that most people have little idea of the permanency of online posts.

"Your footprint is like a digital song – it never degrades. The ones and zeros will always be floating around in digital space."

That point was driven home recently when the Library of Congress announced it was archiving every Twitter message sent – beginning with the very first one on March 21, 2006.

On a numerical scale, archiving all those tweets seems like a herculean feat. Almost 50 million are sent daily, according to Twitter.

It's not really a tall order, though, Bailenson said. At 140 characters or less, all those messages can be stored in a space no bigger than a medium-size bathroom, he said.

And forget about controlling a message once it has been sent, he said.

"People have this illusion you can just delete a message and it's gone," Bailenson said.

That kind of editorial control exists only on your personal computer. Once it is sent out into the digital ether, the information is controlled by anyone who copies it.

"People feel they're bulletproof in digital space, but the reality is that they're being monitored more closely and in a more rigorous way than anything we've seen before in history," he added.

In a twist of irony, many individuals are now opting to meet face to face, rather than relying solely on social media, said Alejandro Reyes, social-media expert for the Sacramento Marketing Lab.

Reyes organizes monthly events in the city called TweetUps. The events draw roughly 200 monthly and allow social-media users to meet – no digital footprint involved.

"I don't beat around the bush – there is no privacy with social media," he said. "A photo I post now will be seen by my grandkids 100 years from now. So you have to learn to live above reproach with what you do."

Doing so may just keep you out of court, too.

Increasingly, what is posted online is being used as evidence in the legal realm, said Michelle Sherman, a lawyer and author of socialmedialawupdate.com, a site devoted to the legal ramifications of social media.

"I encourage people to always assume that what they are posting will go further than that first initial layer," said Sherman.

And that secondary layer might just be used as evidence on court documents.

"Courts are very clear that as far as they're concerned federal rules for discovery include social media," she said. "Courts are definitely admitting social media as evidence in trials."

An endless minefield awaits the careless poster. As a result, many U.S. companies now have strict usage rules, and many are starting to closely monitor what employees do on social media, both on and off the clock.

Data leaks via social media are rising, according to a recent report by the online security and data loss prevention firm Proofpoint. A quarter of U.S. companies that were surveyed had investigated the exposure of confidential or sensitive information, and 11 percent had terminated employees for the violations.

With the rise in leaks, companies are also employing staff to read emails and social-media messages, the report said. A third of the companies surveyed said they now employ staff to monitor email content.

Sherman counsels people and companies to use applications such as Hootsweet, which allows cross-posting of social-media content, enables tracking and makes it difficult to send messages mistakenly.

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