[ElissaShevinsky]

I don't believe that this is simply about enabling advertisers to message users. Until I have more data this is speculation on my part. That said - I believe this is an attempt to expand their nearly-saturated US market by facilitating users to connect with users outside their friend network. Until now this was discouraged and could lead to termination of your account.

More details: There is a new tool tip in my FB account now stating "Your messages get filtered automatically so you mostly see stuff from friends and people you may know in your Inbox. To review your filtering options, click Other > Edit Preferences. Note: These filters replace the old "Who can send you Facebook message"