https://www.bbc.com/news/magazine-21478857
According to this BBC report in 2013, Utd had 659 million fans worldwide. Social media is not the only way to measure fans. And if you've lived in Asia you'll know what I mean. You list Man Utd historical players and miss out on David Beckham, the first global football superstar. I don't know if you are old enough to remember the early 2000s but Beckham was as big a name as Messi or Ronaldo today and his popularity was off the charts, something which also made Man Utd a household name everywhere in the world. You've also missed the likes of Giggs, Cantona and Rooney.
Unless I'm not mistaken Ronaldo only played one season for Barcelona (which was great season for him, but I don't think Barca won the league) and Maradona had 2 unremarkable ones, so I don't know how they become historical Barca legends. Out of those 11 Ballon D'or you mention, Messi alone has 5, which further reinforces my point regarding the Messi effect.
I'm not disputing Barca's current standing, I'm simply stating in a historical sense its a stretch to call them the 2nd biggest club in the world.