Why is Facebook doing VoIP?

Facebook has started to offer voice messaging in some markets with apparently plans to offer VoIP too in near future.

The voice messaging is obviously a smart usability enhancement. Rather than typing text, say something.
The realtime bi-directional VoIP is a different animal.

There are some factors for and against full VoIP by Facebook happening in the markets.

Pro:
+ may generate more service stickiness
+ one more modality to communicate, text has been there quite a while
+ Facebook may quite well be so much more powerful that the operators cannot stop it
+ WebRTC is around the corner and as an open technology may steal the middleman position Facebook has envisioned for itself between companies and consumers. The game around WebRTC is evolving to an interesting one

Cons:
- operators are an important distribution channel for Facebook in many markets (but what can they do?)
- questionable use case.  People may use social networking separate from realtime communications and prefer avoiding phone calls

I believe the key driver is Facebook's will to remain between the consumer and businesses.
This is hard to argue against, one needs only to look at Google to understand why.