Facebook began inserting advertisements directly into users' news feeds today.

There won't be a lot of ads--just one per day--but they will be unsolicited. Instead of being on the right hand side of the page and marked "sponsored," they will appear randomly within users' news feeds and be labeled "featured."

Screenshot by CNET

According to Facebook's Help Center, "Businesses can pay to feature a post so there's a better chance you'll notice it." As an example, the Help Center writes, "Say you like your gym's Facebook Page. Some friends see the story in their news feeds, others may miss it. The owner of the gym can pay to feature the story so your friends are more likely to see it."

This is pretty much the way the "sponsored" stories work, but those are much easier to distinguish since they're not embedded in the news feed.

To figure out whether a post is legitimate or an ad, Facebook users can hover their mouse over the word "featured" and a black box of text will pop up saying, "A sponsor paid to feature it here." The only way to get rid of the ads is to click the "x" on the right corner of each post, deleting them one by one.

Facebook's Help Center also says that ads can be included in various ways, such as when a page you like posts something new or when a friend likes something, check-ins, uses an app, or plays a game. The Help Center emphasizes, "If a story does get featured, it's shown to the same people you originally shared it with."

From 2006 to 2009, Facebook ran ads but discontinued them until last January when it started posting "sponsored" stories on the side of users' home pages to boost brands and businesses. Last month, the social-networking giant announced its plans to place these ads in people's feeds.

It's unclear whether Facebook will continue to increase its advertising presence, but it seems the company is not put off by a lawsuit it's facing in which plaintiffs allege that "sponsored" stories violate a California law that mandates a person's name or photo cannot be in a paid ad without consent.