If you look at the FB phone app after not using it for a week or so, you will have 1-2 notifications. A few hours after opening the app, 7-10 notifications will appear, creating a false sense of urgency to check the app so you don't miss something. Heroin dealers do the same thing when they notice customers not coming around anymore.

The psychological tactics facebook and others use trigger the release of dopamine just like cocaine, heroin, amphetamines, and gambling.