I've gone through the documentation. I haven't found anything that suggests you can edit a comment (or status update for that matter) after it's posted.
Does anyone know if you can?
I suppose you could delete and repost a comment, however it becomes less than real time.
J
I don't think you can edit a comment/status once it has been posted. I mean, if Facebook users can't edit their previously-posted status updates, why would Facebook allow apps to do so?
I know I'm a bit late, but as of May 2014, you can edit a comment in Facebook SDK.
You follow the exact same procedure as in posting a new comment, with the difference that you also provide the comment-id of the comment you want to be updated.
Here's the source from the Facebook documentation: Graph API Comment Documentation