LinkedIn, in contrast to MySpace, is at the other side of the anonymity/inappropriate spectrum. In fact, I don't think I've ever seen a spam profile in LinkedIn. It's all business. In fact, Rupert Murdock's News Corp, which owns The Wall Street Journal (and MySpace) is considering buying LinkedIn.
I like LinkedIn a lot and maintain profiles in both Facebook and LinkedIn. Because of the professional nature of LinkedIn, the profiles there are very complete. Most LinkedIn profiles contain more background information than the average resume does.
The main difference between LinkedIn and Facebook is that LinkedIn is entirely focused on business social networks. Facebook is more general and crosses social spheres. I have friends in Facebook that are actual friends, business relationships, general acquaintances, and people I'm following online.
Another big difference between Facebook and LinkedIn is their advertising models. Facebook is starting to follow a Google AdWords model in which just about anyone can participate. It's very easy to set up an advertising account, choose how much per day to spend, bid on a click rate, choose a set of demographics and interests, and post an ad. LinkedIn has a high-end ad network approach. You can buy banner ads in LinkedIn but they have a pretty high minimum buy. But I'm getting a little ahead of myself. Let me back up and lay the foundations for why so much attention is being given to social media advertising.