While 71pc say that they post only some of their feelings and thoughts on social media, most teenage girls (95pc) keep their deepest emotions out of sites such as Facebook and Twitter, the research for digital TV channel E4 found.
As well as not revealing crushes, problems at home, depression and body image issues on such sites, more than half (53pc) say that they exaggerate on social media.
Over three quarters (78pc) say that they have worries about posting thoughts and feelings online. Around two thirds say that they keep a diary for therapeutic reasons, with 64pc saying that writing in a private diary makes them feel better.
The research was commissioned by E4 to tie-in with My Mad Fat Diary, a new, Nineties-set TV series based on the real-life teenage diaries of Rae Earl.
Earl, whose diaries have been adapted for the series, said: "As long as young people still feel lonely, fat, mad, sad, happy, hopeless, good or bad they will always need a place where they can record their thoughts and work things out in privacy."