using mikrotik as dns server is not enough, many services have hard coded dns servers. Google chrome for example will use google dns regardless of your settings so your only option is to hijack DNS requests. Many ISPs do this as a way to block websites as it is very cheap in resource and admin.
I would not suggest blocking youtube or facebook completely. Many tutorials and video lessons are available on youtube. There are plenty of math tutorials on youtube for example and many other useful and beneficial things. Many use facebook for organising events so thats a positive use of facebook.
If you block these things (whether you are a workplace or school), people will use proxies and such so thats not the best way to deal with it. I would block facebook games and apps (not the phone app) but facebook is also a way for people to get in touch other than email and sms. Many useless and low quality browser games use facebook and are basically free to play but the game quality is so bad that blocking it would be a help (you would also have to block those sites too).
Mikrotik makes it so you cant install any software onto their routers, what some organisations do is just install a linux server and configure it to be a router so they can install customised filtering software which makes it easy to deal with things that reduce productivity. You can use multiple hijacking techniques and force use of a web proxy server (you can actually run squid, squidguard and clamav on a ubiquiti edgerouter which i do) but it may not work on apps. I do know that the facebook app uses web code so it is actually a web browser only pointing to facebook (it explains their high resource usage and battery draining).
If you are a parent and just wanting to stop your kids from wasting time with these things than blocking these things entirely doesnt help, rather you should just block the unhelpful facebook features and perhaps put them on the lowest priority in QoS (or put some bandwidth limitations to slow it down to encourage them to do other things).