Michael - you should have signed off 'yours sincerely'. Use 'faithfully' when the salutation is 'Dear Sir / Madam'. When you address the recipient by their name, use 'sincerely'. If your opening salutation is 'Yo, blood', the appropriate sign off is 'Laters'.

I'm with Jon - this is a conversation in written form, dialogue if you will. It is much less formal than other written forms and other people's 'incorrect' spelling and grammar doesn't bother me in this medium. This is from a person who fully punctuates text messages and never uses text speak or abbreviations. But I don't mind if other people do.