I've actually never run into this! They really say "hihi" or do they just write it? I've seen hehe quite often and the Chinese examples from royal shiny above. Maybe they are using pinyin to sound out the equivalent of "hee hee"?

i am also surprised about this! but chinese people use this! this is like a trend! they use hehe,hihi,呵呵，合合etc to indicate that they are happy or normal but not angry!

I think usually girls like to say that. so hihi = hi, they are just to girly.

"hihi" = "hehe" (sound of giggle)

Hihi is the sound, the artificial sound for a short laugh.