IDC (International Data Corporation) just published a press release announcing its new study called Worldwide Social Platforms 2009 Vendor Shares (if you want to download it, it costs $3000!).
According to the press release, the study found that even though 2009 was a tough economic year, the market for social platforms showed significant gains. In fact, revenue grew 55.9% year-over-year (some vendors grew triple digits from 2008-2009!). The total market size in 2009 was $369.7 million dollars.
Based on market share, IDC finds that the Top 3 Social Platform Vendors are IBM Lotus Connections, Communispace and Telligent. As you can imagine, I'm pretty happy to know that "my " product is #1 
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Take a peek at the press release here.