I think Facebook and Tweeter are both free. Why do they need bitcoin? I don't think receiving bitcoin from big advertisement clients make sense.
En re. Twitter, I was in reference to them embracing it as a payment option via tweets, akin to what some Bitcoiner has already done but on a grander scale. Once the tweeps are acclimated, the currency would be used for any product and service Twitter WILL later provide.
After the IPO, Twitter will be on the crowded battlefield to show a profit each and every quarter to their stockholders, and there's only so many ways to cut the status quo pie, but if they think outside the box it puts the other social media entities in the position of playing defence.