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Twitter has entered the top 50, in at number 42 with an estimated 21.2 Million monthly uniques. Twitter first entered comScore's Top 50 in June '09. Note that comScore probably undercounts Twitter's audience by a lot, because many Twitter users don't visit twitter.com but use third party clients. We reported last week that an estimated 65% of Twitter users still use twitter.com, so it may be undercounted by as much as a third (but it's hard to know for sure, as many people use both the Web and third party apps).
via readwriteweb.com
One of the comments mentioned that comScore doesn't take into account SMS which is a big feature for Twitter. Combine that with the 3rd party services and Twitter is a “big deal.”
