1) used interchangeably to mean either "your" or "you're" by illiterate cretins who can't tell the difference between the two
2) used to mean "you're" by people too lazy to hit the apostrophe key; slightly less reprehensible than the above
1) "youre such a stupid youre stupid falls out of youre stupid." "Ah, the irony."
2) "youre still here right?" "Yes, my apathy-stricken acquaintance."
To spam a worthless forum thread with extremely long and equally worthless posts, ie 100 pages of 50x50 pixel animated burger GIFs. Rarely is actual text used.