I recently became a LinkedIn Convert. 

Although I spend a fair amount more of energy online than my immediate peer group (blogging, etc.) I'm still skeptical at the exuberance others exhibit towards social networking in the business arena. 

I was initially among those who anticipated annoyance at the prospect of constantly receiving "connections updates" from those in my LinkedIn network.  Somewhere along the way, this shifted, and I became a convert.  I was going through a stack of business cards and culling through them.  And I realized that the bulk of the people for whom I had cards were - or at least should be - in my LinkedIn network.  Then it hit me:  I should just throw all of the cards out.    Business cards will probably always be around in some form or another, but once you get to know someone and do business with them, you probably no longer need their business card.   They should be a part of your LinkedIn network.  What this means is that every time they switch jobs or careers, you can just keep track of them through the network, rather than having to get a new card from them.  [I'm sure we're a couple of years away (outside the Bay Area, at least) from people just beaming each other their contact info, but it will probably take a while before the technology is sufficiently adopted for this.  There could still be a paper calling card of some sort.]

So this news that LinkedIn has achieved a billion dollar valuation isn't off the charts shocking.  What's odd is that I've never even noticed any LinkedIn advertising.  I mean to say that it's fairly unobtrusive, which is a great thing.  LinkedIn hasn't touted its ability to integrate its contact list with outlook (etc.) but this could be the next step.  Either way, at bottom, LinkedIn is an easy, paper-less, convenient way to keep track of your contacts. They also make portions of their network available on a paid-access basis, I believe.  I have heard that recruiters and those seeking employment have found success in LinkedIn. Some parts of the business have some potential privacy landmines, but overall it seems like LinkedIn has achieved serious traction with users.  (Part of this could be tapping into the competitive instinct that users feel to generate more connections.)

Looks like with all of the attention, LinkedIn is down today.