Time sinks:
Facebook
Twitter
Stumbleupon

I find LinkedIn is less of a time chewer, of course you can have many hours of fun updating your profile to include just about every facet of work experience you've been through. Creative writing :).

I'm noticing LinkedIn provides some of the same sort of social linking and interacting that Facebook provides. (thumbs up)

One thing it's not providing is sandwich eating updates or how you felt you went with last nights somnolence or anything to do with Farms, unless you are in that sector. Thankfully.

Your LinkedIn profile doesn't do much for you unless you're looking for a job, but it might be useful if you join an active industry specific group because you can interact with people who know their stuff, and they generally aren't likely to tell you they are going shopping this afternoon.