The poll asked 1,479 people aged 14-24 to rate which of five popular social media platforms had the most negative effect on their users.
They were asked to score each platform on issues such as anxiety, depression, loneliness, bullying and body image.
Mental health charities urged companies to act to increase users' safety.
'Inadequacy and anxiety'
The Royal Society for Public Health study says social platforms should flag up heavy social media use and identify users with mental health issues.
The RSPH report warns that "social media may be fuelling a mental health crisis" in young people.
It can also be used as a tool for good, the report said, and companies should be doing their best to make platforms a safe place to be.
About 90% of young people use social media - more than any other age group - so they are particularly vulnerable to its effects, although it is not clear what these are on current evidence.
'Deep depressive episode'
Isla is in her early 20s. She got hooked on social media as a teenager when going through a difficult time in her life.
"The online communities made me feel included and that I was worthwhile.
"However, I soon began to neglect 'real life' friendships and constantly spent all my time online talking to my friends there.
"I fell into a deep depressive episode aged 16, which lasted for months and was utterly horrible.
"During this time social media made me feel worse, as I would constantly compare myself to other people and make myself feel bad.
"When I was 19, I had another bad depressive episode. I'd go on social media, see all my friends doing things and hate myself for not being able to do them, or feel bad that I wasn't as good a person as them."
Social media has also been a positive in Isla's life.
"I have blogged a lot about mental health and I'm quite open about it and have good conversations with people about it.
"I find it gives me a platform to talk and talking with people is something I find imperative to my own health.
"The online friends I made five or six years ago I'm still friends with to this day and have met many of them in person."
The online survey asked participants a series of questions about whether YouTube, Instagram, Snapchat, Facebook and Twitter had an impact on their health and well-being.
Participants were asked to score each platform on 14 health and well-being issues.
Based on these ratings, YouTube was considered to have the most positive impact on mental health, followed by Twitter and then Facebook.
Snapchat and Instagram were given the lowest scores overall.
Shirley Cramer, chief executive of the RSPH, said: "It is interesting to see Instagram and Snapchat ranking as the worst for mental health and well-being - both platforms are very image-focused and it appears they may be driving feelings of inadequacy and anxiety in young people."
In light of the findings, public health experts are calling for social media platforms to introduce a series of checks and measures to help tackle mental health, including:
- Pop-ups warning people that they have used social media for a long time (supported by 70% of young people surveyed)
- Social media platforms identifying users with mental health problems and "discreetly signposting places they can get support"
- Platforms highlighting when photos have been digitally manipulated - for example, fashion brands, celebrities and other advertising organisations could sign up to a voluntary code, allowing a small icon to be displayed on digitally altered photos
The report also recommends that NHS England comes up with a vetting scheme for health and well-being information so young people are better able to judge whether information is trustworthy.
Ms Cramer added: "As the evidence grows that there may be potential harms from heavy use of social media, and as we upgrade the status of mental health within society, it is important that we have checks and balances in place to make social media less of a 'wild West' when it comes to young people's mental health and well-being."
Tom Madders, from mental health charity YoungMinds, said the recommendations could help many young people.
"Increasing safety within social media platforms is an important step and one we urge Instagram and other sites to act upon.
"But it's also important to recognise that simply 'protecting' young people from particular content types can never be the whole solution."
He said young people needed to understand the risks of how they behaved online and should be taught how to respond to "harmful content that slips through filters".
Instagram was contacted by the BBC but did not respond to a request for a comment.