A "fix" is slang for a dose of a drug, such as heroin. Drug addicts say "I need a fix" meaning I have to take some drugs now to satisfy this addiction, otherwise I will go into withdrawal. I imagine the word become common in the 1960s when it seems there were a lot of heroin users around, but it can be used for any addictive substance. The word is now very common and people use it all the time when talking about coffee, for example. "I need my morning coffee fix" is an expression I hear all the time from people who don't want to talk until they have their first cup of coffee. And now with social media, people use "fix" to mean more than just drugs; rather it can be anything that we become obsessed with, such as television, social media, even sports etc. It is interesting that drug culture has hundreds of slang words in English; it is almost like another language. A "fix" and perhaps "blow" (for cocaine) and "weed" for marijuana are probably the most commonly used words. Anyway, I hope that helps.

This 'fix' means something like 'satisfying ones needs/urges' So in the sentence that you have posted, the meaning is something like '...look at their phone screens or social media (to satisfy their social needs)...' It usually has a nuance of just being enough to make do.

Social fix in this example means an action with the purpose of feeling better after doing something (in this case, social media helps people to feel better and go about their lives, it is like a coping mechanism in phycology).

a fixation