I doubt if this will work.... When the free service becomes something you have to pay for, there's always a big challenge. Also, I do not believe that there are no other services that could replace Twitter with equivalent high quality and wide-spread influence. Say, Plurk is a similar mini-blog service that's becoming more and more popular. Additionally, how can Twitter clearly distinguish the "personal use" and the so-called "commercial use" and to decide on charging or not accordingly?
It was only a matter of time before Twitter formally announced plans to begin charging for commercial accounts. Twitter co-founder Biz Stone has confirmed that the heavily-funded startup will begin charging companies for commercial use of the service, MarketingVOX reports.
....The difficulty for Twitter, of course, will be in determining what qualifies as commercial use and what's simply intended for self promotion.