It works very similarly to other forms of addiction.  Facebook "Likes" (or Instagram, etc.) give your brain a little hit of dopamine, and you return to get more again and again. 

https://www.theguardian.com/technology/2018/mar/04/has-dopamine-got-us-hooked-on-tech-facebook-apps-addiction

Now, if you'll excuse me, I gotta go check my Facebook.