Very late reply but just incase anybody else comes across this any is still looking for an answer here are a few plugins that enable you to share your order.
MeanBees Promote/Share order (link)
Nice and simple, does the job in the easiest way possible.
Addshoppers Social Marketing plugin: (link)
This extension gives you loads of additional features plus the post purchase sharing option. You can also use their in depth tracking services to get more information about social sharing that occurs on your site. Oh, and you can also add discount codes for people that share both before and after purchase.