I'm trying to find an average number for the total number of users using social networks around the world (all social networks included).
Look up the UM Wave 3 and Wave 4 reports on social media research in google - you should find them to download as pdfs and there's loads of stats on that sort of thing in them.
Finding the average is so pointless. You need to look at your niche, look at your budget and target your niche. I don't see how knowing that Facebook & MySpace are huge and Hedgehog Rescue Online is tiny so the average is somewhere inbetween will get you anywhere. However a tiny social network might be just as valid and valuable an advertising tool as the big one if you end up getting the sales. It's all about matching your product/service to the audience.
Find an average is impossible....Hope this link will help you Top 3 Social Networking Sites by Country wise ........