I don't use Hi5, don't like MySpace and deleted (or should I say, disabled) my Facebook account. So, why do I use and even like Twitter?
On social networks, I have to post personal information, keep it updated, reply to friends, decide whether or not to install some viral widget... I have to commit to spending time on it.
I don't mean that all of the social web is a time-waster, the Social Web can be useful. The extreme example is possibly LinkedIn, the social network meant for people who don't want to or can't afford to lose time. But to take something useful out of it, we still have to commit.
For me, that's the main advantage of Twitter: it doesn't require commitment!
I can have my client on for a whole week or not even visit the website the next week, because using Twitter is communicating non-essential information.
Twitter can be valuable if you somehow leverage the relations you have with the persons on your network, it's a matter of seizing opportunities. But most of the time you can just do with it something useful, fun, or simply post whatever you want. No commitments.
