There are so many interesting things on the Internet, blogs, social networking. And outside work you may find an interesting home for your pet peeve, political view, or hobby. But the Internet is not really partitionable between personal and work.
My advice, if its personal, no matter how harmless it may seem, keep it anonymous, written under a pseudonym. Why? Because you can make only one first impression, and what impression something may make to your employer or a potential employer may do irreparable harm. Sure you should have freedom of speech and expression, technically, but practically, employers look, and may be offended. Its not right, but it happens. So better safe than sorry and publish under an assumed name.
Your real name is your brand, protect it. Use it only in the most professional way possible, in a way that will promote you as a desirable professional. No politics, no viewpoints, no controversy. Sound boring? It is, but its safe.
A salesman (and that's every job seeker who needs to sell themselves) can sometimes take risks if he knows his audience. He can tell an off color joke or talk about party excess, if he really knows his audience well enough to know they won't take offense. But on the Internet, you don't know your audience, so you need to exercise caution.
LinkedIn is a professional Internet presence. But I've seen people put some questionable personal dialog in their public profile. I'd say that is ill advised. Facebook is becoming a business networking tool. I haven't looked at Facebook close enough yet, but I know that Myspace is just not a good vehicle. Myspace is too social, too informal, not suitable for business unless you are in the entertainment industry. Do you have interests in that area, does it appeal to you? Fine, register under an assumed name, not your real name, not your professional brand.
So sell yourself on the Internet, but express yourself anonymously.