A few columns ago, I argued that in technology almost nothing is ever really “over”. Things are often said to be “dead”, when they are just waning a little.But I have been picking up indications lately that social media, supposedly the lifeblood of the millennial generation, could be at the start of a decline among that very demographic.I recently met a 23-year-old student of English at University College London, who seemed to be one of those smart people instinctively in tune with the zeitgeist because she lives it.I quoted her briefly in a column on Facebook and Twitter video livestreaming. “Nobody I know does it,” she had said. “Facebook is just not cool.”