Pop superstar Christina Aguilera has finally joined Twitter as she prepares to join Maroon 5 frontman Adam Levine and Cell Lo Green as a judge on the NBC show, "The Voice."  Aguilera's handle on the popular micro-blogging site is @TheRealXtina.  "Thanks for the welcome to Twitter. I'm getting the hang of this," Aguilera Tweeted Wednesday afternoon.  "The Voice" is scheduled to premiere on NBC April 26th.