What is a humble brag?

"I am well aware that I am the 'umblest person going," said Uriah Heep, modestly.    
Charles Dickens, David Copperfield
The modern humblebrag has several niche forms. One is the casual reference to showbiz pals/contacts. Here's Caitlin Moran reminding us that she has hung with A listers: 
"I’ve been seen with Lady Gaga and Benedict Cumberbatch. My puppy turns more heads" 

Even better to imply that you are not only on fist-bumping terms with the big shots - they're actually esteemed colleagues. Jake Owens neatly grabs some reflected glory from fellow country singer Blake Shelton. 

“Watching @blakeshelton perform on late night TV right now… He’s a damn fine singer and a great guy. Congrats on the success buddy.”
Another clever humblebrag is to over promote yourself in the celebrity pantheon: 

 This can backfire, of course. What happened when they discovered weren't Mr Bieber, Adam?