Would you like me to prove water is wet and the sun hot as well? Seriously, what happened to history classes in school? As just one of countless millions of primary historical referents, Dickens himself wrote of children as young as eight and nine frequenting the gin shops of London, and artist George Cruikshank created many prints illustrating small children frequenting these bars, such as this:
Countless other diaries, historical articles, personal letters, and the like paint a similar picture. It took me all of thirty seconds to find an 1883 letter to the editor in The Spectator magazine, detailing a mother who fed her infant daughter primarily on "nips of gin".
Tobacco use was as common, stemming from the 17th century belief that tobacco had medicinal properties (it was even purported to prevent the Plague). Historian Pepys writes of tobacco use by young children, and the British Navy of the 18th and 19th century issued a tobacco ration to even their youngest sailors (cabin boys often began at age 10).
You have it backwards. Once alcohol or cigarettes leave a shop, it's impossible to determine who'll actually consume them. But social media accounts are registered to a specific person and. If a minor uses that account to upload images of themself or send messages, that creates a permanent record, and one easily trackable. How many adults are willing to risk a felony charge to allow that?