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Guest blogging is a viable way to build links and authorship and it will always be. You only need to watch out where you post your posts, are those blogs related to the article? Are your posts worth reading and sharing or are you writing them just to get a backlink (if that's the case, that's a complete NO NO). An advertorial has nothing to do with guest blogging. Advertorials are posts (articles or whatever) designed to specifically promote a product/service that is paying for that promotion. It isn't just plain advertising as someone, usually with authority, is promoting your products in exchange of a money compensation. There was a lot of chatter a few days ago when Cutts posted a VIDEO about advertorials, you should watch it and read the comments below (we also published an article about the controversy on Google identifying advertorials that can't really be identify). Hope that helps!