I never say 'lol' or 'rofl' (and feel kinda dirty even typing these out)... but when I do type out something to signify laughter online, I normally mean it. So, if you read a post by me and it's starts off with 'Heh', that normally indicates a slight chortle; 'Tehe' indicates a giggle; 'Hahahahahahahaha' indicates a belly-laugh... and these are all heightened if they're in capital letters. This goes for IMing and texting and anything where I'm not talking face-to-face. But, yeah. When I type a laughter-signifier, normally I'm laughing in real life. I wouldn't want to overuse it, as it would just become another instance of 'lol'. Ugh, I feel so unclean!