Not sure. I studied crowd behavior through sociology when at university and remember looking into the pattern of streaking behavior in the 70s (? - might have been 60s). Typically started at an elite university, then spread to all the 'lower tier' universities in the same region. Wouldn't be surprised if FB took same approach wrt signaling.
They started at Harvard then spread to other elite universities (outside Boston) before slowly opening up access to 'lower tier' universities in the same region as Harvard like you said, then other regions.
[0] Zuckerberg's Facebook started off as just a "Harvard thing" until Zuckerberg decided to spread it to other schools, enlisting the help of roommate Dustin Moskovitz. They began with Columbia University, New York University, Stanford, Dartmouth, Cornell, University of Pennsylvania, Brown, and Yale. From https://en.wikipedia.org/wiki/Mark_Zuckerberg#Facebook
[1] In March 2004, Facebook expanded to Stanford, Columbia, and Yale. This expansion continued when it opened to all Ivy League and Boston-area schools. It gradually reached most universities in the United States and Canada. From https://en.wikipedia.org/wiki/History_of_Facebook#FaceMash
[2] Harvard students are no longer the only ones cyber-stalking their classmates and professors on thefacebook.com. With the click of a keyboard and squeak of a mouse, students at Columbia University and Stanford University can now track down that hottie in section or get help with problem sets. Mark E. Zuckerberg ’06, the website’s creator, opened his online networking service to Columbia last Wednesday and to Stanford the day after. From http://www.thecrimson.com/article/2004/3/1/facebook-expands-...
