Using the phone constantly will drain the battery a lot faster than standby. While this should be a "well, duh" moment, it's worse when the screen is in use. I can't speak for snapchat, but youtube requires the phone to be awake and the screen on. I've noticed that my battery life is significantly reduced when using apps that keep the screen on, whether it's a game or reddit or facebook or or or...
The fact that your screen shows 20% usage tells me/us that is the cause of your battery drain, coupled with the other apps that you're using which have to use the other phone functions. My battery is at 14.5 hours usage, and the screen is only 11% with minimal app usage otherwise. When I'm using it more frequently/consistently, it's not uncommon for the battery to need to bump charged at about 8 hours.