Depends upon your employer, but realistically speaking they'll be able to tell what you're doing and fire you quickly as a consequence. Not only can your employer monitor your internet traffic and see every site you're visiting, but if they have keyloggers then they can tell exactly what you type, and if they have SMS then they can connect to your computer and see everything you do.
You pose a security threat to your company, and your best bet is to just quit your job and find one elsewhere, at a place where they allow you to visit sites unrestricted. Getting fired for circumventing your company's computer security policy is pretty much a guarantee of never getting a decent job again. And if you succeed in connecting to Myspace in violation of your company's security policies and you download a virus or other malware or cause a security breach, there's also the possibility of your (former) employer pressing criminal charges against you.