For Safer Internet Day 2017, we spoke to an expert about what you should do if you see - or are sent - something online that shocks or upsets you.
Helen Westerman is from the NSPCC. Here's what she had to say:
Facebook says that safety is always their priority and that they have a range of tools on the site to help people to keep safe.
However, they point out that you are supposed to be 13 to sign up - and they will remove your account if they find you have not told the truth about how old you are.
Instagram says they also have tools to help people keep safe and happy online.
A spokesperson said: "If anyone sees anything which makes them uncomfortable on Instagram, they can easily report it from within the app by clicking on the three dots icon (which looks like this '…'), which is next to every account, photo or video."
They also made it clear that you are not supposed to be on the site if you are not 13, though.
Snapchat did not comment.
To find out more about staying safe on the internet, click here.