Due to fact that WebmaisterPro is user content driven social network, we've received warning in Google Webmaster Tools on 20th of March 2014. Obviously user generated content was not the main reason. Many members of WebmaisterPro submit quality articles linked to official sources, unfortunately many submit “garbage” blog posts, with main purpose of link building, even probably link selling!
Actually, I have figure out that something is not quite right on 18th of March (2 days before receiving penalty). Practically, I was already prepared with appropriate action. On 20th of March, few hours before I read the news regarding MyBlogGuest, I used to know that Google will not stop here and actually WebmaisterPro is one of the probable targets.
We've got the warning message and 5 minutes later, I've pushed WebmaisterPro “security” update and submit reconsideration request.
It was quite strange that only the website PR was affected, but actually all SERP was standing, even during the Google penalty period, SERP were intact and WebmaisterPro even achieved higher rankings (probably due to another algorithmic update)
Be sure that you follow strictly Google guidelines and always take appropriate action without panic. At first place be sure that you are not trying to cheat the system, in my 15 years long career as SEO, I have seen many penalties, but even penalties for black hat techniques are not as severe as “semi-whitehat” link building (this also include link selling and buying).
Our guest blog post guidelines are NOT absolute, but are good reference point for guest blogging. Google is not against you, but in war with low quality content. You should also read - "Google is Right To Penalize Guest Blogging. Don't Blame Google and Matt Cutts", to better understand how to continue with guest blogging and keep good standing with Google webmaster guidelines.
We've recovered from this penalty in 5 days!