O.K. your link just went to the cover, but I think you're talking about the 'sharing' feature via FB or Twitter.
You don't HAVE to do it. BUT, when you get to the end of a book on an eInk Kindle -- like all the way to the end, after all the 'end matter' and stuff -- you'll see a page that says something like "rate and share". You can rate it and that will save your rating in "your collection" on Amazon -- which is useful for keeping track of whether you've already read something or not. Or you can simply share that you finished it without a rating.
Sharing means you have to have linked the Kindle to FB/Twitter -- that's done via settings. The post just says you finished the book, though you can add to it if there is room (limited to the 140 characters). When you go to FB you'll see the 'finished it' post with a link to the book on Amazon. Not sure what happens on Twitter as I don't tweet.
If you select 'rate and share' then the star rating you gave it will be part of the message that's posted.
I think it's pretty cool and have had more than one of my friends comment on something I posted this way. . .either that they read it, or were going to or had been thinking about it or whatever.
As this is an Amazon function, it will go whether you're connected by WiFi or 3G. If you're not connected when you do it, it prompts you to turn wireless on. I think if it can't get a connection right away, the post will just not go through.
Interestingly, it's not available if you read the book on the Fire.
