Today on Twitter, Justin Bieber was feeling a bit nostalgic. The now 17-year-old superstar tweeted an old video of himself singing 'U Got It Bad' for Usher, when he was just barely on the cusp of puberty and fame. Usher shot back with a tweet of his own, letting the Biebs know that he can live anything he can dream. Elsewhere on Twitter, other music stars were tweeting similarly positive tunes. Paula Abdul, Diddy and Alicia Keys each shared a little wisdom with their followers, while 'Glee' star Darren Criss was coming down from a flashmob high and 'The Voice' stars Dia Frampton and Adam Levine were ... Well, who knows what they were talking about! See what your favorite artists were chirping about today!