Many clients are now investing time and money on keeping Twitter and Facebook updated so with all of that do you still need your blog updated? Absolutely!
If your blog is installed under your own domain and resides on your website server then adding to your blog benefits the organic placement of your website as you build blog content. Twitter and Facebook are important ways to engage customers, search engines are starting to look at your activity on these networks as part of their SocialRank scale which impacts organic placement, but activity on these platforms does not build website content like blogging does.
When it comes to choosing where your money is best spent to improve organic search placement I like blogging best, then Facebook and finally Twitter. I place Facebook above Twitter as Facebook is where your prospects are spending a significant part of their time and I feel it is important to engage them where they are active.
Although SocialRank does not carry the same weight in organic placement as PageRank, both Google and Bing are actively now monitoring SocialRank. I feel that over time the activity you have on Facebook and Twitter will become more important in affecting your organic placement and where you appear in personalized search results.
In fact for national businesses involvement and engagement on Facebook and Twitter may be key to mitigating the focus of localize search results in organic placement that Google and Bing are both pushing at this time. I say that as search results are now personalized and focus heavily on showing results in your geographic area, but also include a social component where personal connections and interconnections are a factor of the results you see as well.