blabbermouth

blabbermouth    [Blab-er-mouth] 

noun.  a person who talks too much

"Twitter now allows users to chat privately without feeling constrained by a space limit of 140 characters.  Blabbermouths, however, will have to cut off at 10,000 characters."

"Apple employees operate under a secrecy policy so severe that it makes  NSA agents seem like loose-lipped blabbermouths."

"His blabbermouth aunt can't stop talking about her gossips when someone tries to tell her something important."