It looks like Twitter or most likely some third party Twitter services have been hacked. The message “hey! 23/Female. Come chat with me on my webcam thingy here www.chatwebcamfree.com” was found in the update stream of many people. The messages seem to be from some adult webcam site. When I searched for the term on Twitter search, I found a number of tweets with the same thing.
This is most likely an attack on some third party twitter services, the ones that ask for your username and password and let you Tweet directly on Twitter. Some malicious bots might have gained access to the username and password database of the third party service and used them for tweeting the spam message. As a result, many people found that their favorite ones whom they were following were inviting them on live webcams via their tweets.
Twitter seems to have removed this thing now as the last tweet with this spam message was tweeted 3 hours ago. If any such message is in your twitter stream, you should immediately change your password.
In the future, such bugs won’t exist probably as Twitter is constantly working on developing and improving OAuth. OAuth is an open protocol that make secure API authorization easy from third party desktop and web applications. So, even if you use third party applications, you won’t need to share your passwords with them. Anyway I suggest you use Twitter directly rather than other third party services to protect your twitter account from getting hacked and to ensure maximum security to it.