In search of the elusive magic number...

So I've been a member of the Xbox Live community on and off since I first signed up as a beta tester; I have had a Facebook account for a couple of years; in March, I posted my first tweet; yesterday, I became a member of the LinkedIn community; and now I'm writing my first blog. All this has got me thinking...what's the magic number? Although I feel like I'm on social networking overload, I know I've only scratched the surface. Digg, Youtube, Delicious are all out there, waiting to be tapped. I mean, I understand that the whole point of social media is to connect with as many people as possible, using whatever channels are available, but surely there's a point where you start to get diminishing returns or find yourself spread too thin. Where is the balance - that magic number? How many different communities must you join to maximize your reach, while still keeping things manageable?