Though the biggest milestone made in the past month would have to be Facebook's having reached 500 million registered users, that's not the only social network showing healthy signs of growth.

Usage of the main Twitter site has grown by 109% in the last year, according to comScore, with 93 million unique users visiting the popular microblogging service in June 2010, versus 44.5 million in June 2009. 

Regionally, South America and Asia Pacific saw the biggest growth. The former blew up from 4 million users in June 2009 to 15 million in June 2010 (305% change) and the latter similarly grew from 7 million to 25 million.

As far as main site penetration, Indonesia, Brazil, and Venezuela top the list at 20.8%, 20.5%, and 19.0%, respectively.

Because comScore's data does not take into account Twitter access via third-party applications like Tweetdeck and Seesmic, whose usage rates are not usually insignificant (at least domestically), the report does not portray fully how much Twitter has grown in the past year.

Nevertheless, with well over 100 million users (if we take into account those extra applications), Twitter has affirmed itself as one of the most popular social networking sites on the Web. While only a fifth as large as Facebook, the microblogging site, along with MySpace and LinkedIn, has become an invaluable source of information sharing. 

As noted by Graham Mudd, comScore vice president, search & media, nearly 75% of global Internet users access social sites each month, "making it one of the most ubiquitous activities across the Web." With numbers like that, it's hard to imagine there only being one dominant service in the social space.