This is really the root of it - Twitter and Facebook are designed for sharing. Take the photo of the Starter Set dice I posted this morning - that spread pretty quickly, and it showed up on the EN World news feed.
Forums are better for in-depth, back and forth, but they come at the cost of reach.
That's why we watch the forums to see what's happening and what people are talking about (along with Twitter and Facebook), but use streaming video, social networks, and our own web site to put information out there. It lets us reach as many people as possible in both directions (talking and listening).
Plus, from my POV it's much easier and more efficient to spend 15 minutes a day replying to questions on Twitter, then stepping into the middle of a forum conversation that I may or may not actually be able to contribute anything useful to.