> then you'll only show one angle of your life, and the rest will simply become invisible to facebook.
I feel like the opposite may happen? I.e. instead of only showing one angle of their life, the idea is that people share _all_ angles of their lives. Either way you end up with the same result "one persona".
The only people who care this deeply I think are the same ones who block ads or don't use facebook anyway.
Letting people show different angles of themselves through differently branded websites makes it much easier for facebook to build complete, thorough profiles of people.