A friend sent me a card and I sent him back saying thanks. He replied saying 'Glad you liked the card' Why didn't he begin the sentence with I'm or I am....
Is that common or is it a typo?
The linguistic term for that is ellipsis - omission of some words from a sentence in -as Mike said - informal communication. It's really very common:
Happy to be on an island in the sun. = I am happy...
Don't know (Dunno) = I don't know.
Satisfied? = Are you satisfied?
Never been there. = I have never been there.