This is highly dubious because football is so parochial and tribal. Just because lots of people follow him on Twitter, it doesn't mean that they're going to rush out and buy a Juventus shirt. I think Messi is the best footballer that I've ever seen, but I've never bought a Barcelona shirt in my life, and never would. Equally, Twitter is a very poor barometer for popularity. Lots of people follow Piers Morgan, but I'd hardly describe him as popular. For some bizarre reason, people follow him so that they can abuse him.
Most football supporters don't like Ronaldo. It's really only Man United, Real Madrid, and Portugal fans who like him, and now Juventus. Are Real Madrid fans going to rush out and buy a Juventus Ronaldo shirt? No, they're not. Most of them will already have forgotten about him, and will be more concerned with whether or not they're going to sign Hazard. It's only really a delusional tranche of United fans who still like him, because they've bought into his quite transparently insincere public pronouncements, and believe that he might return to Manchester one day, which he obviously never had any intention of doing.
The only people who are going to be buying Ronaldo shirts are kids, Portugese people, and Juventus fans. No-one else gives two hoots about his move to Juventus.
In terms of being the most popular sportsman in the world, I would say Roger Federer is far more popular, as he's almost universally liked by everyone who has even a passing interest in tennis, whereas far more football supporters dislike Ronaldo than like him. I respect him as a player, but even when he played for United and I used to watch him in the flesh, he was never a favourite of mine, because he was quite obviously always about one thing and one thing only (in a team game)...the person who he sees in the mirror.