Many of us use the "like" button on our friend's posts and even when visiting company sites.
If you click on a "like" button for a store or brand, Facebook plans to incorporate your "like" into generating a "Sponsored Story" for advertisers that want to pay for it.
Once you "like" something, Facebook does not offer a way for you to opt out of your "like" being featured in an ad.
The one limit is does have is if you have set your privacy settings for your posts and limit who can see them, when the company buys a sponsored ad, only the people you authorized to see that original post can see it.
"Facebook to let advertisers republish user posts", Barbara Ortutay, Associated Press, January 26, 2011.