I get asked that question pretty regularly. I am active here on Active|Rain... I'm even an Ambassador. I'm linked up on LinkedIn. I'm showing my face on FaceBook, with 4 business pages and a personal page. I am a solid light on Flickr. I tweet on Twitter, Digg and find things del.icio.us. I am also on about 50 other networks to a lesser extent... from music to Jeeping and sports cars.
Why do I need a blog since I am using all of those other tools to interact with consumers?
Well, for one thing there is only so much you can tell someone with 140 characters, a picture or a song. It is great to give people links to cool stuff, but it is better when some of the cool stuff is your own, too...
Simply put, a blog is the centerpiece of a complete social media strategy. Aside from being the repository of your best material, it can also be a collection point for tweets from Twitter, links from Delicious, wall conversation from FaceBook, information from LinkedIn and all of the other social information we are generating.
Also, as the centerpiece, it is a way to broadcast that great material you are publishing on your blog. Linking back to posts on the blog in Twitter, FaceBook, LinkedIn, Delicious and more is a great way to get the word out... Let someone know that you have written the ultimate post on _____.
And there is yet another level to the synergy. And that is profiles... Many social sites, like Active|Rain, allow users to link back to their websites. And many of those, like Active|Rain, allow Google to follow those links. So, by having a lot of powerful networks linking to your primary site, you can deliver higher page rank to it.
Your blog is like the cake. The other social networks are the decorations. Without the cake, they are kind of pointless. But, the cake without frosting and sprinkles is not the same, either.
Sometime we'll have to talk about RainMaker blogs, WordPress and other options that are out there for the socially active agent. They aren't all the same, and some options are better for some than others.