Last March, NASDAQ-listed communication and collaboration software maker Mitel Networks (Mitel) hit Facebook with a patent lawsuit. Now, the social networking giant is retaliating with a patent lawsuit of its own, with a few interesting twists.

According to the complaint (docs embedded below), Facebook is alleging that the Canadian company infringed two U.S. patents (7,778,396 – “Telephone status notification system” and 7,454,709 – “User definable on-line co-user lists”).

Interestingly, both those patents were acquired by Facebook for $550 million in April 2012, about a month after Mitel filed the initial lawsuit.

The seller was Microsoft, but the Redmond software giant had just acquired the patents from AOL in a transaction valued at $1 billion for the entire portfolio.

So basically Facebook got sued by Mitel, bid for AOL’s patents, lost the auction to Microsoft, ended up buying a portion of AOL patents from Microsoft, and is now countersuing Mitel alleging the infringement of two of them. Are you still with us?

Among the inventors listed on the patents-in-suit are folks like Barry Appelman, who is billed the father of the “buddy list” and AOL instant messenger, Yossi Vardi, the affable Israeli uber-angel investor, as well as former ICQ CTO Yair Goldfinger (Mirabilis, ICQ’s parent company, was sold to AOL back in 1998).

Aside from countersuing Mitel using these old AOL patents as weapons, the social network giant has also filed a motion to dismiss Mitel’s initial lawsuit, which is currently pending before the Delaware court.

Mitel, meanwhile, was recently forced to slash 200 jobs and close some facilities after reporting lower than expected revenues for the quarter ended July 31, 2012.

We’ve reached out to both Facebook and Mitel for comment.

Facebook vs. Mitel Networks