This has happened twice to us, that we've noticed. The most recent was a couple weeks ago. We had a group of people in our home, some were on phones/iPads, not browsing though. FB wasn't open. Husband went on the computer later that evening and an ad for a single handed comment he made, was right there on FB. He'd never searched for it before, had just made the one-off comment.

I fully believe that's what they are doing. It happened before, but at that time I dismissed it, though it did creep me out.