Some great points on why Asynch Communications scale against Synch communications. Couple of points I can add quickly.
1. Asynch communications are long term. You can write an article or a blog post today and have someone comment on it year later.
2. You will add more value to the conversation if your platform supports elongating the async communications. YouTube does this well. Twitter doesn’t. Twitter communications are short lived and since the tweets are not track-able later, the value users put in these communications diminishes with time (and this time is essentially 1-2 weeks). If you look backwards 6 months and see your Twitter stream, how many of these communications actually make sense to you now. But do the same on YouTube with comments around a video, it all makes sense.