Is Twitter Peaking

On his blog Micropersuasion, Steve Rubel asked the question whether or not Twitter is peaking. He thinks it is. My opinion is that there is still tremendous capacity for Twitter to continue growing. Just think about brands you buy products from. Are they on Twitter? wouldn't it make sense for them to engage customers through that network? As long as we can point to capacity, I think it's hard to make the case for it peaking. Thoughts?