Google has gone after yet another guest blog network that is being used by SEOs for link building purposes. The link network is named PostJoint and has gone on record that they are different from MyBlogGuest, which was penalized a month ago.
Matt Cutts confirmed on Twitter in a round about way that PostJoint and likely all the publishers in the network, were penalized. I bet they will start seeing unnatural link notifications within Google Webmaster Tools Manual Actions Viewer this coming week.
Google's algorithms are currently unable to detect the difference between genuine and spammy content marketing activities. Platforms like yours mess with our business model leaving us with no alternative but to setup a complex and labour intensive manual operation. As a result, Google has de-indexed your site.