I don't see the big deal about T witter reaching a critical mass. It's like any other new technology or product - at some point, pretty much everyone who wants it will acquire it (not talking about expensive luxuries). User disinterest is expected after a period of time. The novelty wears off, and the loyalists maintain a certain level of usage.
The only issue I see as important is if usage drops enough to negatively effect monetization, which in T witter's case, isn't an issue yet. Then again, people who do T witter regularly tend to do so frequently (does that make sense?). An acquaintance of mine has 3 accounts, and updates each one at least 10 times a day. Amazingly, at least half of his updates contain interesting information, compared to the drivel that a lot of people post. Once they figure out how to monetize the product, I don't think that they will have a problem. As I said, the loyalists will keep it going.
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