Privacy on Twitter 
Protecting your updates (tweets) is completely pointless.
As Twitter penetrates more and more into our working lives, it is important to understand how the mechanics of your tweets are handled by Twitter as you may end up regretting some of your updates.
Protecting your tweets
The whole point of Twitter is to reach that audience with your message. Most people do not follow people that have protected their tweets. You are therefore not reaching the potential audience that you would have done. But that is not the only negative; Google will not have indexed your tweets either so any potential SEO benefit is lost.
In any case, even protected tweets are visible to those who know a little about how the internet works.
Using XML is just one way…
Example: To see anyone else's private, friends-only messages, pick one of the user's friends, and then substitute their user name in this URL:
http://twitter.com/statuses/friends/[username].xml
Blocking someone
Blocking someone means that your tweets and picture will not appear on the blocked party’s profile page, friends time line, badge, or anywhere else. The person will not be notified that they’ve been blocked, and they will be unable to follow you. If your account is public, the blocked party can still view your profile page, but can’t receive your updates in their timeline or on their phone.
When you block somebody, they can still:
- Read your timeline
- Send you @replies, which are still visible to everybody else, and remain within Twitter search, and will be delivered to you if you have a search for your replies configured on Seesmic Desktop or TweetDeck
- Re-tweet your messages, which can give the impression to others that you are ‘friends’
If you’ve had experience as a forum administrator, you’ll know that when you properly block somebody, you have the facility to stop the person from reading anything on the forum. With plugins, you have the same powers when you run a blog.
Similarly on Facebook, once a person is blocked they can not see anything on your timeline (assuming you have correctly configured the privacy settings.
Privacy
When someone follows you on Twitter, your tweets appear on their timeline and if you follow them, theirs appear on yours. As you gain more and more followers, you are able to reach a wider and wider audience, an audience that you probably would have never been able to reach before.
Even if they are not following you, your tweets are visible to everyone who cares to visit your twitter page. It is like having a phone conversation where you don't know who is listening in.
It is important to realise that this medium is a public conversation not a private one. Treat everything you say as if you are posting it on your web site, because it is. It can be seen by anybody.