Are you going to refuse a link from your boss ? You could refuse one from a co-worker, but that will inevitably lead to workplace unpleasantness, so you accept anyway.
I got enrolled because my bosses did, then I got invitations from all of my colleagues, and now I'm getting invites from ex-colleagues.
It's all workplace-oriented and you don't dare decline if there's the slightest chance that you'll be seeing the person again.
LinkedIn is the ultimate extortionist network.
I can't wait to find out when they'll start peddling ads or stuff to "monetize stockholder investment".

* Does LinkedIn have 12M or 15M users (both figures in the article)?
* Is LinkedIn making a profit or still hanging on the lifeline of venture capital (both statements in the article)?

Nasty bit if those corporate releases are inconsistent, huh?