Let's see... FB launched the mobile "share" feature this December. Twitter launched retweet in 2009. AFAIK tumblr has always allowed reblogging. Finally, Soundcloud just enabled re-posting this December--it used to be a primarily "create and upload" community but now you can repost someone else's stuff.
Don't get it twisted. I like Instagram the way it is, and it may never allow it. That said, I think eventually they'll bite the bullet. My guess is late 2013 to mid-2014. What do you think?