Twitter allows you to mark certain posts as favourites.  You may do this for a number of reasons - because you thought it was funny or interesting, because you want to reply to it later, or as a general bookmark feature

They're Private, Right?

Wrong. You can see any Twitter user's favourites - even if they've protected their tweets. You can't see the tweets of protected users.  Here are my favourites http://twitter.com/edent/favorites

You can also see who has marked your tweets as their favourites.

What Do Your Twitter Favourites Say About You?

Let's take a "random" example - The Royal Mail.  They only have one favourite.

🎧 phantomile poro 🎧

@CardCaptorKaren

It is written into the CWU constitution that Royal Mail must go on strike once every 4 months. Whiny fucks.

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Nice.

If you're advising on social media policy - be sure to tell your clients that what they mark as a favourite are open for all to see.  What they bookmark may not create a favourable impression.

Now, there may be perfectly legitimate reasons for clicking "favourite" on an obnoxious or offensive tweet. You may want to mark it so you can show your lawyers, blog about nasty people on the Internet or even a simple mis-click with the mouse.  But in the highly fraught world of social media - it pays to think twice about how your actions may be perceived by others.