By all accounts, Twitter's doing what it's supposed to: coaxing more people into using its high-speed social snark service... just maybe not as fast as some would like. According to Twitter's third quarter financials -- in which the company met or exceeded most milestones Wall Street types were hoping for -- 13 million people started using the service over the past three months, bringing the total to a whopping 284 million monthly active users. Not too shabby, right? That's up pretty substantial over last year, too. The potentially hairy issue is that Twitter brought 16 million people onboard last time it dropped numbers, and 14 million the time before that. Couple that with the fact that Twitter expects to rake in slightly less money next quarter than analysts expected, and you've got more than a few people concerned that Twitter's growth is winding down.