A Facebook content moderator discovers a photograph online that may hold the clue to uncovering the mystery of her niece who went missing years previously.

"When he discovers a photograph that may hold the clue to? his nieces abduction, a Facebook moderator must..." (Then tell us what he must do)

And then?

What you have given us is the setting, now tell us what happens. What is she going to do with the photo? Give it to the police? Hire a private detective? Investigate her niece's disappearance herself?

Check out the logline Generator (see link at the top of the page) to understand what information a logline must contain.