They said they'll use it to support their 'personal analytics' programme [0], which is free via wolframalpha.com - I don't see how this data would help with Mathematica or anything else they charge for?
Even though WA is closed source and for-profit, I few them (and the company) as a kind of fellow scientist and not the evil big corporation (like oracle, microsoft etc.).
[0] http://www.wolframalpha.com/facebook/