Unanswered questions about Twitter’s progress

Twitter co-founder Biz Stone put out his first e-mail newsletter of the year on Wednesday, trumpeting the progress his microblogging service is making.
But it was short on useful statistics and metrics.
Stone notes that in the last year registered Twitter accounts have grown more than 1,500%. But he doesn’t state how many accounts there are.
Online analytics company RJMetrics estimated in January that Twitter ended 2009 with just over 75 million user accounts.
However, a large percentage of Twitter accounts are inactive. About 40% of accounts have never sent a single tweet.
Only about 17% of registered Twitter accounts sent a tweet in December, an all-time low, RJMetrics reports.
A number of independent researchers have pointed out that Twitter’s growth stalled around mid-2009.
In the company’s blog, Twitter analytics lead Kevin Weil says the service is seeing 50 million tweets per day. He claims that “tweets from accounts identified as spam have been removed,” but what about the rest of the spam that’s on Twitter?
Also, what percentage of those tweets are “retweets”? My use of retweets has jumped significantly since Twitter made retweeting as simple has clicking a button.
Studies have shown that most tweets on Twitter are coming from a small percentage of very active users. That makes Twitter sound like a niche, rather than mainstream, service.
The nagging issue of Twitter’s ability to make money off its service appears to have been quieted by the company’s recent partnerships with Google, Microsoft and Yahoo. Twitter has licensed its full feed of all public tweets to the big three web portals. Twitter describes this as its “fire hose” of data.
But other questions about Twitter's business remain.