I once saw a similar ad on my Ubuntu machine, telling my I had been caught by the FBI with child porn. My computer was supposedly "locked" as punishment. The ad said that content was located in my C:\Users(this part was blurred)\Desktop folder.
I knew this was BS since, if the FBI actually did believe I had child porn, they wouldn't just lock my computer as punishment. Simple logic says that possessing something so disturbing wouldn't be punishable by a slap on the wrist, nor would they allow me to bribe my way out. Rather, they'd likely kick down my door and arrest me.
PS: Ubuntu doesn't have a C:\ drive, nor does it have a folder called "Users" (instead, it has /home/(username). Additionally, my computer shows no sign of being "locked".
I do hope the FBI finds whoever posted this ad, and arrests them for impersonating law enforcement.