It is so simple, too. You pick a story you want someone to comment on - any of your stories. Then you pick the type/rating of a story you want to leave a comment on (i.e. True Story, Fan Fiction, or Original and G, PG, PG-13, R, or NC-17).
Mibba will then give you a link to a story that you must leave a comment on. The only rule is that your comment must be 200+ characters (not words!). This is only slightly longer than a tweet... not too hard to do!
Once you've left your comment, you can do it all again! Fun, right? I think so. :)
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What do you all think of this feature? Have you tried it yet? How many comments have you received/given? Will you use it again? Will you use it frequently? I know I probably will. :D