Bill Clinton to George W. Bush: Why aren't you on Twitter?

Bill Clinton has one question for George W. Bush: How are you not on Twitter?

The wisecrack was embedded in a Presidents' Day greeting Clinton posted Monday on the social media site. In the post, Clinton gave a shout out to President Obama, who has been praised for his Internet savviness, and former President George H.W. Bush, who joined Twitter in December. 

The message quickly went viral and even earned a retweet from Obama, and more than 6,200 others. The hashtag #HowAreYouNotOnTwitter even became a trending topic in the United States for 10 minutes, according to Trendinalia, a site that tracks popular hashtags.

Clinton only created a Twitter account last year after a public nudge from comedian Stephen Colbert. Bush might be dragging his feet by avoiding Twitter but he does have a Facebook account, which has amassed more than 3.1 million "likes."

The last time all living former presidents - Clinton, the Bushes, and Jimmy Carter - were all together was in April for to commemorate the George W. Bush Presidential Library and Museum. Beyond that gathering, George W. Bush has largely avoided the spotlight since leaving the White House.