You're spot on, this is exactly the issue there was. I'll explain what happened on our end.
In our last version of Wysija we added the View in your Browser link, with that we thought of giving a pretty url to each email's browser version so we've created a a custom post type named wysija. And then on update we had a procedure to insert a custom post per email, it could be many depending on how often you were sending. The function we were using to achieve that is wp_insert_post, now we've realized that in that function can be hooked other plugin such as automatic post to facebook or twitter. This is something we were not aware in our testing environment we didn't have any auto post to facebook plugin installed as in a testing environment you don't really want to post to social network all the garbage you might create.
Anyway that's not everything, this issue of crazy numbers was happening only on some server where the update procedure was failing and repeating itself(hence the humongous number of posts). This is an issue we've discovered few hours after the release on last friday. We've corrected it minutes after we've realised the importance of the issue.
Now our lesson is learned, and we will never use intensely a WordPress function which might be hooked with other plugins and which can potentially do all sort of things.
So rest assured Arthur, this will never happen again.
Though as I was pointing out to another one of our users, I think it would be good if this kind of auto post plugins by default were not posting automatically to all kind of custom posts unless authorised in its configuration. So this issue will not happen with wysija again that's for sure but then nothing protects you from other plugins using the wp_insert_post function to insert hidden custom posts.
My apologies for all the time you might have spent cleaning your Facebook.
Now you can update safely to 2.1.2 :)