Someone "tipped" them an acorn, usually to applaud a particular post.
It's also a past way to send acorns, but it's public. If you click on the number, you see who snt the acorns, and how many. Also, you don't get an automatic Tree-mail to kick off a conversation.
http://garden.org/thread/view_...
Post #9 on this thread, about 70 posts ago.
