Shortened words are never really 'allowed', people are just lazy. People only ever use things like 'r' and 'u' when texting, because phone keyboards aren't very convenient. They're never correct, and it looks horrible when I get messages from people saying something like "how r u?". Things like "haha", "hehe", "huh", "um" "ah" are used when chatting because people say them in spoken conversation. These aren't words, but actual sounds that people make when speaking. People also often write shortened words or emoticons to express emotions, like "XD" and "lol". A lot of shortened words are in the spoken language, like "math/maths", "pic", "gym" and "fridge" instead of "mathematics", "picture", "gymnasium" and "refridgerator" respectively.