In terms of twitter, you can make your own profile private and then only your followers that you approve can read your tweets. There are also ‘direct-messages’ that you can send to people privately which are similar to private messages on other websites.

There’s a general unwritten rule of thumb that you shouldn’t put anything online that you wouldn’t want to be public knowledge. Most people don’t understand this. Today, e-mails can be hacked and ‘private’ data can be made public and therefore it’s important that not just on twitter, but anywhere online that private information/thoughts/pictures aren’t posted unless you’re ok with them being available for the public to see.

In my opinion, the power of twitter doesn’t lie in sharing information with friends. It really lies in the learning from those in a position that you value.

Twitter is owned by a single company. If you’re curious about it’s infrastructure and terms, it’s important to read over the terms of service on the twitter website.

Twitter does have a standard API that developers can use to put various tweets and twitter data on their own websites. Currently, there are hundreds of companies that use the twitter API and more people use twitter API based software that isn’t affiliated with the company than there are people that actually go to twitter.com. Tweetdeck is an example of this http://www.tweetdeck.com/

If I should not use it for private info with my friends,
I might consider it as tool for Theme-oriented chatting,
where every “channel” (unilike IRC) can “listen” to another and get only relevant info?

Also, one more thing I never used it till now is:
It twitter owned by only one company?
If Only one company controls servers, infrastructure, control mechanics of news,
could it, just as easy as receiving messages, do some “censorship” in time of needs?
What law does twitter needs to serve?

I see that there is no: Privacy, Company-independent service, platform-neutral service, public standard for service and even promise that service as it is would continue to be available as it is.

If twitter should be a network, it should have a standard implementation,
independed implementations that you could put on your private server, etc,
and not controlled as network from one identity, company, country of justice system.
If it is not all of that.. why bother? Marketing? Ah well, thats great marketing tool as I see.