Uh, you probably don't want to tweet to @POTUS, actually

Twitter, at first glance, is a magical fantasy land - a utopia where any old egg can freely converse with presidents, pop stars and kings. It is the great democratizer; the global water cooler; the one place where you can say anything to anyone and know it'll be OK.

Consider this your monthly reminder that this is actually NOT the case: On the modern Internet, impressions of anonymity and ephemerality are, well ... usually fake.

Case in point? After Barack Obama belatedly joined Twitter on Monday - in his official, presidential capacity - dozens of Twitter denizens began tweeting him sex jokes, threats and other unprintable inanities. (We counted nearly 500 tweets dropping f-bombs at POTUS in the past day.)

But the joke's actually on them: Not only does the Secret Service already monitor Twitter for threats, but the White House is archiving each and every thing @POTUS tweeters say. On Gawker.com, Ashley Feinberg flagged this relevant piece of the White House Online Privacy Policy:

"On Twitter, the White House automatically archives 'tweets' from official White House accounts, 'direct messages' sent to or from official White House accounts, and 'mentions' (tweets from other users to official White House accounts) ..."

In other words, when you tweet "leave Michelle for me @POTUS," or something even more inappropriate/creepy, that lives in an official White House Archive for eternity (slash, for potential future scrutiny).

To be clear, this isn't just a White House thing: The principle applies, in one form or another, to most types of social networking. Some personal data, once uploaded to Facebook, never really disappears unless you close your account. Tweets, even after deletion, can live on in caches (and elsewhere). Even your Google searches - which feel so fleeting, so meaningless, in the moment! - can be archived and aggregated and, potentially, subpoenaed for use in court.

"Just seconds after posting something online it has likely been disseminated to dozens of people and definitely been archived by an unknowable number of automated systems," the programmer Pehr Hovey wrote in his master's thesis on deleted tweets in 2010. "Though a delete button may provide solace to those having second thoughts, in reality it is a fade" - you can never take back those @POTUS tweets.

• Dewey writes The Post's The Intersect web channel covering digital and Internet culture. http://www.washingtonpost.com/news/the-intersect/