Figuring out the value of a Facebook fan has become more complex for many small-business owners, ever since the social-media giant began asking businesses to pay to "promote" their posts.

Under a program rolled out in May, businesses pay Facebook Inc. anywhere from $5 to hundreds of dollars to promote a post to the news feeds of users who have "liked" their page, plus Facebook friends of those users. The price depends on how many users a business wants to reach.

The...