Being that my business is a promoter for other local small business, talent and non-profits I have actually set up to where I key into facebook and then it automatically sends it to Twitter. I am trying to hit as much of the social media channels as I can but reading through the discussion thread, I am seeing another view point.
I currently use Facebook for both of my businesses. I prefer Facebook esp for promoting my tax & bookkeeping business. It has a better reach, and there is much more to see and experiment with as far as advertising and marketing go. Easier to network and navigate. Twitter just seems too jumbled together, condensed, and convoluted for me. It's like logging into one long traffic jam w/o much to look at (except endless street signs very easy to get lost). I have integrated it into my Facebook page for my small boutique shop but use it only to update/tweet new merchandise listings, if it weren't for the button on the shop site though I probably wouldn't have even bothered. I am gearing up to do the same for my tax and books business (like a tax tip of the day or something) so I can take advantage of every free avenue available but still prefer Facebook. I also use LinkedIn and I'm starting to like it too.
Twitter is only as active as you make it. If you post useful information daily, then it can be a very effective tool. If you spam constantly with useless information and follow unrelated topics, then you are simply wasting your time.
Facebook is a 1 time set up, that sits out in the world of the internet forever and can be searched at any time.
Just an opinion coming from the head SEO specialist for a small business:
In my opinion they are both necessary, they offer such different ways to interact with both current and potential clients. With twitter you can search for someone who is looking for your services, and engage them. From there you can carry on a conversation and build a relationship. With facebook people have to find you but it is hard to argue with the traffic. With facebook you are more of a brand presence since posting more that a few times a day will irritate fans. On twitter you can broadcast since it is normal to post 10 + times a day.
AS a few people mentioned, there are tools (I really like hootsuite) that make using both really easy. With that in mind just remember that both platforms provide an outstanding way for business to market themselves to users and offer first line customer support. Not using either would to be do you and your business a disservice
Hope this helps,