Wow, I'm always suprised when people feel so strongly about facebook! Like, it's a page on a screen. When DH & I deactivated FB a few years ago,his family cried, lol. For reals. Like we died or something. We were like 'It's ok, we're still here'.
I think feminism has nothing to do with it. How do you feel if people have joint email addresses? Does that fire people up equally? What about joint postal addresses? A lot of people are just not heavily invested in *facebook* as an important part of their identity. For a lot of people, it's a basic communication tool, just like an email address or even postal address. Not a symbol of their relationship and how controlling it may or may not be.