Although we haven't literally reached the point where, as Andy Warhol predicted, everyone achieves world fame for 15 minutes, the nature of viral photos and videos does technically make that possible.
Not only could the most unexpected things make potentially anyone known to millions of people, but that fame can often be fleeting and contingent one one specific thing they did.
To use an old example, there are likely a lot of people in this world who would stare blankly if you said the name Tay Zonday, but whose eyes would flash with recognition if you instead called him "the Chocolate Rain guy."
So while we may never know the people behind these suddenly popular photos, they definitely stuck in people's minds for a reason.