(Paul Morse/George W. Bush Presidential Center/Getty Images)

Its Presidents Day, and at least one actual U.S. president didn't forget.

Bill Clinton, who is quickly leaving his predecessors and successors in the dust when it comes to social media, tweeted out a Presidents Day greeting on the social media site this afternoon.

And in the process, he ribbed President George W. Bush for his absence on Twitter with the hashtag "#HowAreYouNotOnTwitter?". Bush is the only president since President Reagan not to be on social media.

The younger Bush's father, President George H.W. Bush joined in December.

Happy #PresidentsDay - to #44 ( @BarackObama), #43 ( #HowAreYouNotOnTwitter?), #41 ( @GeorgeHWBush), & #39. #PresidentialTweeters

- Bill Clinton (@billclinton) February 17, 2014

President Jimmy Carter, who Clinton referenced as #39, has also skipped social media-at least for now.

So far, none of the other presidents have responded to Clinton's President's Day greetings.