Alcohol prohibition was not a universal failure - alcohol consumption was way down during prohibition, as were alcohol-associated issues such as violence and domestic abuse (drunk driving wasn't really as much of a thing in the early 1900s). But, there were other consequences from alcohol prohibition, such as organized crime, bootlegging, enforcement issues, lost jobs and businesses, and lost tax revenue. Now, after the end of prohibition, there's lots of tax $$ coming in and lots of jobs in alcohol, but also lots of death, illness, and other negative societal and family problems. Alcohol perhaps causes more death than any other legal substance in the U.S. (maybe tobacco would be first, I don't know the current numbers).
In any real-world situation, causality is really complex and lots of factors interact with each other. BUT, I would note, we still prohibit children from purchasing alcohol. And I see no reason to not prohibit social media for children.