LinkedIn could be turned into the world's best job board. But it is currently built like an old-fashioned dating site. It's like Match.com for jobs.
I'm not going to say the valuation is right or wrong... but LinkedIn is very valuable for me: No more carrying around business cards I get from other people and I can easily keep in touch with people in other companies in my industry and see who's hiring/changing positions.
I consider Facebook the new AT&T and LinkedIn the new Rolodex.
However, $8B for what essentially amounts to a glorified harassment tool for recruiters is a bit ludicrous.