One time, not too long ago, it used to be the button fans clicked on when they wanted to "become a fan" of someone popular on Facebook. It soon evolved into a button that people clicked so often, out of their own will, to become a fan of something they used to (or still) do.
Following the huge success, fan pages had a great downfall. Pages that required people to become a fan to see a 'funny' picture emerged out of nowhere like ants at the sight of your picnic. "Become a fan" has hence become synonymous to "you'd better fan this page if you wanna see what it's all about"
Back in the days: Bob became a fan of Miley Cyrus.
Soon afterwards: Bob became a fan of I used to do backflips while taking a shower.
And now (unfortunately): Bob became a fan of Become a fan if you want to see how rocks used to look before they became famous!!1! :O