The presidential campaigns of President Barack Obama and Republican challenger Mitt Romney are collecting huge amounts of data about voters' browsing habits in a bid to target their online ads to the most receptive Web users. That's led to concerns about how the campaigns will use that data once the election is over, with some privacy advocates saying the data is likely to be acquired by private companies. "We simply don't know how this information is going to be used in the future and where it is going to end up," warns Christopher Calabrese of the American Civil Liberties Union.