Based on what I've heard, it was just a rumor. Originally it was just for laughs but then some people mistook that information and created a rumor out of it.
Totally an hearsay, I can't find any reliable source that tells facebook is starting to accept bitcoin very soon. 3 years after OP asked this question still no one gave a source that prove that Zuckerberg is considering bitcoin integrating to his business. But it will be a great adoption if he'll start to use bitcoin for facebook as it's now on the trend and bitcoin now is very popular.
Facebook like microsoft or other large companies that do not want to use something they can develop on their own, there is no reliable source that discusses that FB really wants to use bitcoin, I'm sure they try to make their own coin algorithm for their payments, if using bitcoin too fast then FB will get a negative response from many countries, especially developing countries that still consider bitcoin as research studies.
They can develop their own coin or payment method just like what Alibaba is doing now, they have alipay and Jack Ma thinks that bitcoin is nothing compared to his invention. That coin that facebook will make is like something that can be bought to them directly and can only be used for purchasing their ad spaces / sponsorship ads.