Facebook has allowed you to edit your comment for a couple of seconds after you posted them. This functionality has been available to a larger number of their users for a while now.
However, they are now rolling out an update that allows you to edit your comments at any time.
Sources say that the company also provide access to the history of the edited comments so the viewers can check out what has changed. This is a good feature to have.
Google Plus has allowed the users to edit their posts and comments since the very beginning. Twitter of course has no editing capability.