Next time you sign in to Facebook, take a look at those three or four little ads on the right side of most of the pages. Do they mean something to you?

One might be for a wedding photographer, another from a political candidate, another for a company offering to publish your book, or trying to sell you sleeping supplements.

Chances are that at least one or two will be targeted to the activities and interests you post on Facebook, or the city you live in, your gender, or even your relationship status. These little ads are typically purchased through Facebook's "self service" system, which enables small- and big-time advertisers to create an ad in minutes to lure specific demographic groups with a few lines of text and a graphic or photo.