forums for YouTube and Twitter?
Twitter in particular... I don't see how that particular format would help you out, unless you were perhaps live-Twittering a conference. You wouldn't even be able to fit your message into the 140 characters allowed for a Tweet - but then again, you've got room for Twitter-esque abbreviations
As for YouTube, it requires a lot of effort. I'd love to do YouTube, but for anything that's going to take anyone's imagination, it takes effort. Mere expounding in front of the webcam risks obscurity. A good, rehearsed script at a minimum, or else an insane amount of editing - in which case you might as well cut the video together as though you meant it that way (Lyndon Headroom, anyone? :)
QualiaSoup and AronRa are two of my personal favourites over there. Actually, I'm minorly ashamed to say that I do not follow cosmology on YouTube - I'm sure there's something
there, though :)
I just wouldn't consider any of those replacements for forums. Twitter is good for status updates, and YouTube is good for short comments, but not much in the way of discussion unless someone actually makes a video of their own in rebuttal, and that's pretty rare.
YouTubing, if you can put in the effort, is still a lot more likely to find success than a podcast would.