Hi there puzzlenutty and
to ATP!
I'm not sure what you mean by favorite but if you want to let them know you think they have posted a good photo of plant(s) or written plant information that is helpful to you, you can click on the green thumbs-up in the lower right-hand side of the box in which they posted the photo or comment.
You can also give them acorns which is the currency on ATP. You can read about acorns here:
http://garden.org/special/acor...
If you are meaning favorite as in like on Facebook, someone else will have to help you with that as I don't do Facebook.
Again,
and we are so glad you're here!