It is incredibly unlikely that FB could actually litigate this - it falls under the same legal doctorine that protects ad blockers. Also, a few years ago there was a company called Sambreel that had a product called PageRage. They modified FB pages and inserted ads. Facebook never sued them. In fact, Sambreel actually sued Facebook [1] because the only thing that Facebook could do was pressure advertisers not to work with the company [2], and the company felt wronged. Their lawsuit against Facebook was dismissed, but Facebook was never able to do anything legally against them. One interesting quote from the lawsuit:

"...between July 2009 and October 2010, PageRage grew significantly, and was regularly garnering more than 1 million users per day, and generating more than $1 million in monthly revenues....by mid-July 2011, PageRage had grown to more than 4 million users per day"

In this case, however, there aren't 4M installs. We're talking about less than 300K installs. Facebook would probably not bother with such harsh tactics here.

[1] https://www.scribd.com/fullscreen/114953759?access_key=key-1...

[2] http://adexchanger.com/online-advertising/sambreel-locked-ou...