Froggy, the reason I think there's nothing sinister about the start date on the 'Where's William?' FB page or the date the photo was taken is because I completed a unit on Forensic Imaging at Uni which included studying digital photographs and the corresponding chain of evidence.
The FF's phones, computers, digital cameras, etc, would have been gone over with a fine-toothed comb by forensic investigators; including imaging and computing experts. If, after those examinations, it's good enough for DI Jubelin and Strike Force Rosann investigators to conclude William's FF can neither be named as POIs nor as suspects; as has been stated consistently over the course of the investigation into his disappearance, then it is good enough for me. That is, unless substantially contradictory information comes to hand.
If you, or anyone else, honestly feels the FB page anomaly would be 'of interest' to investigators, I urge you to contact them via Crimestoppers.