It could have been your plug-in. You know how some people and some "people" love seeing blogs that allow do-follow links in their comments. I know that you really want to have TRUE traffic going to your site, and not just some "people" trying to force their backlinks in the comments section. Another reason would be what the others here have mentioned. You've been hit by Google's web spam update, aka the Google Penguin. There shouldn't be a problem for your status on Bing because they have yet to release a black and white animal that would "kill" spammers.
As regards Penguin, the original and the refresh were on known dates so you can track back to see what the effects were (if any) immediately afterwards. But if you were being plagued by spammers and have now gone nofollow, now that the spammers are presumably reduced wouldn't you expect less traffic?
There could be 2 possible reasons for this that plugin or the recent google update- Penguin.
1. First of all De-activate the plug-in & see what happens.
2. Check the rankings of your keywords, how much down they have went?
The OP never asked about SERPs, it was only the traffic they are concerned about. Which does beg the question, was there a corresponding drop in SERPs?
Yeah I think the OP means a drastic drop in the SERPs.. but I don't think that kind of a plugin can interfere with your traffic!
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The OP also has another thread started 10 minutes before this one:
Originally Posted by sparior
I recently got suspended from Google Adsense because of landing page violation.. So I contact Google and it has been 2 days now, I haven't heard from them.
Any tips or ways I can get back to Google Adsense?