555 means lol in Thai for text conversation.

Five (5) is pronounced "Ha" in Thai. So 555 sounds like "Ha, Ha, Ha." I sometimes write 555 at the end of the a joke or I response to the funny stories by sending 555. 

One time, I wrote an email to an American friend and I added 555 at the end of the letter. He was confused and thought I wish him a was wishing him good luck. After I explained him what 555 meant, he has been started using it and also told other friends to use it. 555