This is the method that my team mate, Maya, passed on to me the other day. It uses Photoshop. If you don't have that particular program, I'm sure the method will translate pretty easily to other applications used for editing photographs.
First, get the Treasury in question on your screen and reduce it in size until you an see all of it. On my computer, with the Chrome browser, that is about 75%.
Then you need to print the screen. On my keyboard, bottom left, is a key with "Fn" in a box. Hold that down and press the one up the top, right hand side, that says "prt sc" in a little box. It doesn't say it's done anything but it has saved it to your clipboard.
Minimize your browser and open Photoshop, choosing the editing bit. Click on File, New and Image from Clipboard. The treasury will appear in the editing window just as it looked on screen.
Up top on the tool bar, choose "Layer" and "Flatten Image". Mine is an old version of Photoshop so yours might say "Merge Down" or similar.
Next step - crop the treasury so you can still see the title but you cut out all the Etsy gubbins. Then go to Image, Resize Canvas and change the width to about 1 cm more than the height.
Save it where you can find it again with the appropriate name. You can then upload it to Facebook just like any photograph. Don't forget to add the link to the treasury as the image won't be clickable.
Here is one I prepared earlier. (I've always wanted to say that!)
This is a lovely Treasury made by my team mate Kellie. I am in it, of course! Nothing like a bit of shameless self promotion! It is called An Eclectic Collection of Beautiful Items. Go see it!
If I've missed anything or you need any help, just leave a comment and I'll come back to you.