If you want statistics about social media, here are some useful source sites:

• StatCounter: Shows trends for popular sites for selected date ranges, countries and different statistics (including Social Media).  Based on traffic to the specific sites.
• Internet world stats.com - world internet use and population statistics
• CheckFacebook.com - number of users on Facebook (select country from the map).  Includes demographic segments
• Another source of Facebook statistics by country: www.socialbakers.com
• Sysomos - January 2010 report "Exploring the Use of Twitter Around the World" showing NZ's position at number 20. Sysomos' site contains a lot of other Twitter statistics of interest.
• AUT and ICDC's World Internet Project reports on New Zealand's use of the interenet - includes interesting statictics on uses across cultural groups, where people are using the web etc.
• According to this video by Catalyst 90, we're "Better" at social media than anyone else (another Kiwi Urban Myth by the sound of it) - the music and graphics are cool tho.

 Other sources of statistical information if that what floats your boat!:

• Hubspot - with 120 charts and graphs - focus is on inbound- marketing.  Includes lots of Twitter and blog related statistics.
• EConsultancy - short article with commentary on use of social media in Australia and New Zealand

A word of caution:

Have you heard "If Facebook was a country it would be the third biggest in the world" ? 

Usually quoted by social media evangelists. But it's a a little bit meaningless, because you could probably throw around equally impressive stats for use of Microsoft products, smart phones, drinkers of coffee - you get the idea. 

You have to do your homework and work out if YOUR audience is there and whether they'll engage with you on said platform.