I logged into Facebook today to find my friend's 5 days old post on the top of my newsfeed. It wasn't particularly popular so I was curious to see why it had resurfaced nearly a week later. Turns out it was "Sponsored". I've included the post below:

I headed to the Facebook Help Center to double-check what this meant. I found the following.

What are sponsored stories?

Sponsored stories are messages coming from friends about them engaging with your Page, app or event that a business, organization or individual has paid to highlight so there’s a better chance people see them.

Facebook Help Center

So there were two possibilities here. Either my friend had paid to surface this post using the new Promote feature, or someone else had paid Facebook to feature this post. My friend denied spending any money to promote this. This is where the mystery begins.

Sponsored Stories aren't new to Facebook. When my friends "like" or share a post on some business's fan page, it is often featured prominently on my newsfeed. This falls under them "engaging with [a] Page, app or event", just as the Help Center says. I've included an example of this below:

But take a look at my friend's post again, at the top of this article. He wasn't engaging with any Page, app or event in the sense one might expect. He shared an Amazon link to a product and casually mentioned the product's name in his post. According to Facebook's description, there was no reason to believe this post could be targeted by advertisers.

Additionally, there is no way to know who had sponsored this story on his behalf. It could have been Amazon promoting the link to the product page. Facebook has been detecting/de-duplicating newsfeed links for a while:

It could equally have been Microsoft promoting his mention of "Halo 4". Facebook has already been doing some non-trivial topic detection on post content:

Unlike sponsored "likes" and shares, there is no way to discern who is sponsoring these plain, old Facebook posts or why they were sponsored. The fact that companies can target a user's arbitrary content is a little creepy.

Many people might be concerned that companies can "detect" what they are talking about and use their words as advertising. I personally do not mind. But the Help Center description of Sponsored Stories is outdated at best and evasive at worst. And I think people deserve to know who is benefiting from the content they post.