Answer:
The instantaneous messaging system in most social media apps allow people to communicate quickly, causing people to be more engaged in thinking longer and stay up later. This negatively effects the brain as you are getting less sleep.
Social media apps also provide constant mindless entertainment and losing track of time is a very easy thing to do. This contributes to less sleep.
They can cause you to be depressed if you're constantly seeing people's success when you yourself aren't successful. Or, if you always see people who you think look better than you, it can cause you to feel self-conscious.