Note: We think that what's happened is that the old 'Messi' Facebook page was transitioned across to a new URL (Facebook.com/LeoMessi) and fans from his old board transitioned too. There are also lots of other Facebook Messi pages like this one and this one.
Update 28th April - As of today he has 10,257,816 fans - so he has been adding approximately 1m fans a week, and these figures are real.
"I’m happy to get a few new likes a day on my Facebook profile, but not Leo Messi. He joined Facebook just a few short hours ago and now has over 6 and a half million fans and he’s not done yet. Just in the process of writing this article he’s already jumped up another 10 thousand.
Messi’s Facebook page was flooded with fans after he joined the site this morning, getting over 12,000 likes on his first wall post alone. It’s unclear at this point why Messi fans are flocking to his page in such numbers but at this point he actually has more fans than his sponsor Adidas who only has 1.8 million fans! Currently his page has over 6.6 million Likes in the just over 3 hours that his page has been active."