Short answer, early to mid 2010s
long answer, much like asking when was the first human evolved, when did cars become common place or when did capitalism become the dominent economic system there is no exact answer because barring wars and natural disasters which still have hidden chains of events to cause them most change is gradual. In the case of social media it started with some universities or businesses getting facebook pages, and making the site bigger so more actual people joined which made a feedback loop of people being willing to join, and when people joined more of their limited social time was spent on these websites where conversation was faster and images and video could be added, meaning if you weren't there you had less of your friends time to be with them wich made you want to join and continue the cycle for your other friends who now find you're busy with social media. think of it like boiling a large pot of water, sure the metal at the bottom soon reaches 100 degrees and this makes some bubbles but at what point is it boiled when is enough of the water hot enough there's no clear point.
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