Sarah Lacy asks the question, that Walt Mossberg should be embarrassed for not asking:

So if it’s so great, why only 600 advertisers?

For the record, Mossberg decided to respond to the 600 advertiser number from Twitter CEO Dick Costolo with the following:

Echoed Walt Mossberg, it’s “the perfect wet dream of every marketer.”

Walt Mossberg: asking the tough questions… wait.