If what you say is pure speculation I would very much like to see some evidence from you because Facebook said nothing to that effect. Zuckerberg reaffirmed several times that they intend to work seamlessly with any and all e-mail out there.

This seems to me to be a long overdue and obvious overhaul of their crufty messaging system based on the realization that many people actually do not use e-mail anymore and rely instead on Facebook’s woefully inadequate messaging tools.