Remember that they don't have just one brand on Facebook. For example, they have 1.3 million followers on their Chevrolet page. They have 1.1 million for Cadillac. 400k for Buick. 660k for GMC. 525k for OnStar. 350k for their "TeamChevy" Chevrolet racing page. Now, there's sure to be some overlap there; but that's still easily 2 or 3 million followers.
That's a factor of 10 more than you were estimating; and that's just based on a quick poll of brands that I took a look at.
That's a factor of 10 more than you were estimating; and that's just based on a quick poll of brands that I took a look at.