Lots of businesses have found value in using Twitter to tell customers, potential customers and business partners about what they're up to, where they are and what's new with them. Effective use of Twitter can help build loyalty and inform about your brand but it's important to remember that there are still some 'no go' areas that make tweets less than sweet!
The problem seems to be that when it comes to social media we're all inclined to forget the 'rules' that apply in other areas of marketing. And make no mistake - using Twitter for business purposes is marketing. By its very nature, social media doesn't carry the conventions that we probably take for granted in other forms of marketing; it encourages us to write as we would speak and this informality can mean we get a little carried away. For example, it's unlikely you'd take out a half page advert in the local press trashing your competitors but, for some, this type of message seems acceptable behaviour when tweeting. Whether it's a print advert or a tweet, a "look how awful they are" approach to marketing never works!
In the same way, don't cross the divide between work and social life. It's fine to invite your customers to join you in the Kings Arms after work but it's not so good to rave about the bar in Spain that you visited on holiday, nor to tell everyone what an enormous hangover you had the following morning!
Use tweets to inform, inspire, reinforce your culture, build your brand values and build your reputation. The message here is very much "if in doubt, leave it out".