I also posted the other day a garden.org link into a facebook post and it appeared with a large Pinterest-type P also. I thought that was odd!
Here is a cutout of my post, showing the P! This was a post to the American Orchid Society page. I assumed next time I need to use the official fb link?
Come to think of it, it didn't happen this morning. I posted the link to the Orchid bloom thread for May to my fb page and it posted normal, without the P.