This will not look good on LinkedIn's (NYSE: LNKD) resume.

Shares of the corporate-minded social-networking website fell by more than 5% yesterday after The Wall Street Journal reported that Facebook (Nasdaq: FB) will launch a jobs board this summer.

LinkedIn bulls argue that this is old news. There are already plenty of Facebook apps that detail job listings, and any effort to aggregate the information is more evolutionary than revolutionary. However, there's also a good reason LinkedIn came under pressure yesterday.

Armed with more than 900 million active users, Facebook could become a threat to most online companies. Unlike LinkedIn, where only a sliver of its registered users frequent the site on a regular basis, more than half of Facebook's active users are on the site in any given day.

There's already a fair deal of real corporate networking taking place, and anyone who's been on the site long enough has probably seen more than a few friends or family members secure new jobs simply by communicating through status updates.

Facebook isn't trying to be LinkedIn, though. There are no recruiting fees collected or premium subscriptions to sell for enhanced access. Bears have to give LinkedIn some credit here. It didn't become a $10 billion company -- and, unlike Facebook, a recent IPO that's actually trading well above its initial prize -- by accident.

However, it's also naive to think that Facebook's move may hurt more traditional employment hubs including Monster Worldwide (NYSE: MWW) -- which slipped 3% yesterday -- but not LinkedIn. If Facebook does make improving the way its hundreds of millions of users ferret out potential job openings, no hiring hub will be safe. 

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