It's because it forces you to pick. Outside of the discrete friendship groups online, there's an ever-shifting on-the-spot calculation about who's around you and how much you want to say. One day you might feel like telling friend X while you're in the coffee shop together with just one other friend, the next day you might not feel so open in the bar for a variety of reasons. Add in all the variables about who else is around, how much beer you've drunk, whether you've just been paid, if the relationship with a partner is going well etc. and every situation is different in a very nuanced way. I'm not on Facebook now, but when I was I rapidly gave up on the idea of administering my friendship groups because it felt like I was bureacratising my friendships in a very unnatural way.