At the time that conversion pixels were pulled from the FB toolkit in 2010, conjecture on "Why?" ranged from "data leakage," to legacy Beacon concerns, to not wanting to support the Google display ad ecosystem - which includes pretty much anything beyond the Facebook login save for a few, well-known portals. "Make your landing pages on Facebook!" seemed to be the idea.
But, no more.
Hypothetically, if you were registered on Amazon, Amazon could take your email and ask Facebook to match it to its database of emails that Facebook users use to access FB. Sure - everyone doesn't use the same email address for Facebook as Amazon - but many likely do and that's enough to provide a unique opportunity to address customer audience using the direct marketer's first-party data (email, in this case).
Think of the implications here. Salesforce could offer its customers the opportunity to leverage CRM records and, assuming each record has an email (or phone numbers) with the appropriate permissions, retarget the user on Facebook. Acxiom, Experian, and Epsilon all get this. So do many others in the ecosystem - agencies, ad networks/DSPs, trading desks, etc.
Online-offline, across digital channels - the connective tissue includes the email and the pixel. And, conversion pixels are another example of Facebook's quickly spreading, ad strategy as the promise of addressable advertising takes another step.