When it comes to expressing something via any form of writing, text or chat, I do it in full complete sentences especially when expressing love. By this I mean, I don’t do ‘IMY’ or ‘ILY’ or ‘ILYSM’ or worse ‘ILYSFDM’ (?) . What’s all that ?
It is always best to say you miss or love someone verbally and face to face. That way, we see the eyes and the tone of the voice. Through that we see his/her sincerity. In written there’s no voice tone, neither the eyes. Just letters. So to me it isn’t sincere when someone short formed his/her expressions or feelings.
Maybe it isn’t a big deal and sincerity is subjective. But the same thing like say it like you mean it and in this case, write like it like you mean it. If your feelings are true, why not put a little effort to write ‘I love you’, ‘I miss you’ instead of ‘IMY’ or ‘ILY’. That short formed feelings don’t even mean anything. Besides, non short formed sentences or words are always sweeter. :)