Sarc, I know, but its just Twitter! It could/should have been "UR" or "YR". Who cares?
I'm glad NOBODY here ever makes that mistake !
You're right. With the character limits on twitter, proper grammar and spelling is nearly impossible. I use TweetDeck to manage my accounts and that has an option to auto-shrink what I write to optimize for twitter character limits as much as possible. That can result in some you're/your type things.