"Facebook used to be more of a social network for finding people you don't know, and now it's a social network for the people you know."
That's because within the first couple years they figured out that over 90% of their users were primarily using it to connect with people they already knew. (And these were the folks who were both early adopters and college students.)
Also, if you design a social network around meeting new people then that network will cap out relatively quickly and die due to 'triadic implosion'. This is where person A is friends with person B and C. Once person B meets person C, the open triad becomes a closed triad. I forget the exact percent, but once the closed triads are more than a certain percentage the entire social network dies. However, networks that are designed to connect you with your existing friends don't run into this problem.
Also, designing the social network around your existing friends is more conducive to generating lots of new content cheaply and often automatically by using exterior behavioral residue, which is essentially what this new update is all about. Originally Facebook was designed around the existing social science combined with Zuckerberg's intuition, though at this point Facebook is mostly designed around its own proprietary social science that has been created for internal use. In the short run they are maxing out their stickiness before the IPO, though in the long run it wouldn't surprise me if they ultimately undermine their original appeal by straying too far from the basics.