Re-implementation means that they would be second or third to market. You have to be first in a new category, and Instagram defined the mobile/social sharing space.
You only have to be first to get the network, and they already have the network.
Ebay tried building a Paypal killer and was not successful even though they owned the network. They ended up buying Paypal and killing their own offering because no one was using it. It has been said many times now that buying Paypal was one of the best decision by Ebay management.