The pressure for the "perfect online life" is being blamed for a nation of "deeply unhappy" young teenagers.
ChildLine says many are dealing with fears and worries that didn't exist 30 years ago.
In 2015 the charity says family relationships, confidence and unhappiness were the biggest issues for young people.
Bullying and self-harm also featured prominently.
"I use social media but that just makes me more depressed," one girl said.
Overall, 35,244 of the counselling sessions held by the NSPCC-run service in 2014/15 were related to low self-esteem and unhappiness - up 9% on the year before.
There has also been a rise in the overall number of sessions the helpline runs each year, going from 23,530 in 1986/87 to 286,812 in 2014/15.
NSPCC chief executive Peter Wanless said: "It is clear from the hundreds of thousands of calls ChildLine receives that we have a nation of deeply unhappy children.
"The pressure to keep up with friends and have the perfect life online is adding to the sadness that many young people feel on a daily basis."
One 13-year-old told a counsellor: "I hate myself. When I look at other girls online posting photos of themselves it makes me feel really worthless and ugly. I'm struggling to cope with these feelings and stay in my bedroom most of the time."
And a 12-year-old said: "I feel like crying all the time. I'm constantly worried about what other people are thinking of me and it's really getting me down.
"I use social media sometimes but that just makes me more depressed as I hardly have any friends online and no one likes my posts/photos."
The NSPCC said that the way that children contact ChildLine has also shifted.
Fewer than one in three of ChildLine's counselling sessions were via telephone last year, the charity said, while 71% involved email or online chat.
Dame Esther Rantzen, founder of ChildLine, said: "I am shocked by the acute unhappiness and loneliness that afflicts so many young people which means that, for many, the only place they can find comfort and protection is from our helpline."