Recent numbers from comScore indicate that Twitter is still growing at a hefty rate. From that study, the numbers for June show that 93 million Internet users visited Twitter.com, which is a 109% increase from last year.
International markets seem to be a big driving force of Twitter's recent growth. Asia and South America particularly are areas of big gain for the micro-blogging website.
International markets: largest growth countries:
This study only includes users who access the service from the website twitter.com, but users who access the service through third-party sites or software via the API are excluded.