Facebook, Twitter, Blogs, Forums : which one you will use to promote your Business ?

I think a good strategy would consist of a mix of 2 or more channels, based on your business needs.

All of the above in order to generate as much traffic to my site and as much interest as possible in my business.

In addition to the Social Media sites you have listed above, you can also add YouTube, Answers etc… These are called "Promotion Channels".
Using one of them or all depends on Target Audience and the message you want to deliver to them.
In another words you can decide which of them is the most appropriate by using the below Formula:- "Your message" should be added on the appropriate "Promotion Channel" in order to be delivered to the "right audience".
The type of message (Text, Video, Image) determines the promotion channel, for example: Recipes sites depend more on YouTube when promoting Videos, Recipes, while it depends more on women's forums in promoting text recipes.
From my experience, stronger promotion strategy means having all (related channels) in varied proportions in order to have a higher reach.

No one can ignore any online channel, but we've to be a ware of each channel, as there will be advantages as well as disadvantages.
I think forums / blogs nowadays are a risky channels, you can't prevent any user to write any negative opinion or any complaints, and if you want to remove it, it will take time to be removed from the channel itself and then from search engines, where in the another channels you can hide/delete what you want.
If that happened ( Negative comments ) may affect on your Brand via search engines, where those negative comments will appear in Search Engines ( Google, Yahoo, Bing ,,, ) I think most of us was searching for a brand, and found a negative opinions / complaints.
Think before ...

It's depend if you are targeting consumer or business