This rumor is complete non-sense, Facebook is already present in more cellphones than any browser will ever be, via Java, including all non-smarts, almost all lower-ends and even Chinese copycats support FB trough Java, so it is stupid at best to think FB needs any browser, not to mention one that has less than 2% percent of market share.
I checked all links provided and everybody pointed to a “close trusted source”, what kind of BS is that?