Eh, that's a reasonable mistake to make.
Old People Facebook
The sublemmy for "Old People Facebook" is a curated space showcasing the charming, confusing, and often hilarious social media endeavors of the older generation. From accidental memes and cryptic status updates to endearing attempts at using modern technology, this sublemmy celebrates the unique ways seniors engage with the digital world.
For anyone who doesn’t get it, Facebook was founded in February 2004, initially limited to college students, and not opened to the general public until September 2006.
If you’re curious, see History of Facebook on Wikipedia.
I think you're missing that this was posted to a Horticultural Society, which the user has probably actually been a member of since 1992, he just didn't join the Facebook group back then on account of Facebook not existing yet.
You’re probably right. I was focusing on the fact that it’s a post on oldpeoplefacebook, and didn’t think of that.