A portmanteau of my and celebrity. A term used to describe people who are famous relative to the speaker, but who may not be a mainstream popular figure. Examples could be football players playing for a lesser team, an actor in an unknown or obscure TV series or film, an alternative or non-mainstream comedian, authors, critics and politicians, particularly on a local or regional level.
A: "Hey, I got a tweet off a mylebrity the other day."
B: "Who was it?"
A: "Oh, it was Martin Riley, a defender who plays for my favourite team."
A: "Got a mylebrity autograph the other day."
B: "Oh? Whose?"
A: "It was John Lloyd. He's a television producer and author of some books I like."