Most people have realized that there was an update yesterday/today with google. After researching it for a few hours, I realized that 80% of the blog networks I checked were just de-indexed. I encourage you to check the google cache of your backlinks from networks and you will see what I mean. As always, the SERP changes happen for the next week as your backlink profiles are re-adjusted. If you are falling in the SERPS, this is most likely why. Everyone who uses blog networks, please post your own research of the de-indexing!