First, some housekeeping: Every Wednesday night, we run the report on what stories were most shared by LinkedIn’s 135-plus million members. That means the news of Facebook’s coming IPO — announced Wednesday, Feb. 1 — had only one day to attempt to rocket to the top of our rankings. A Mashable story about the IPO came close, but didn’t have enough juice to unseat an infographic looking at the rise of Pinterest. That’s fitting, actually, as the top two stories show how fluid the current business world is. Facebook is the giant today. But new platforms like Pinterest are popping up, rising and falling, and no business model can go long without finding itself at risk of being upended.  Here’s the list:

Top 5 most-shared articles on LinkedIn (Jan 25, 2012 – Feb. 1, 2012) Follow @LinkedInToday

  1. Pinterest Becomes Top Traffic Driver for Retailers (Mashable)
  2. P&G To Lay Off 1,600 After Discovering It’s Free To Advertise On Facebook (Business Insider)
  3. Forget Networking. How to Be a Connector (Entrepreneur)
  4. Facebook Files for $5 Billion IPO (Mashable)
  5. Web economy in G20 set to double by 2016, Google says (BBC)

The best example of the upending is Business Insider’s look at P&G’s decision to slash ad spending, realizing that it doesn’t have to buy media the way it used to. In its earnings call, CEO Bob McDonald laid it out:

I believe that over time, we will see the increase in the cost of advertising moderate. There are just so many different media available today … One example is our Old Spice campaign, where we had 1.8 billion free impressions and there are many other examples I can cite from all over the world.

P&G is the world’s largest advertiser. When it declares a shift in thinking — and starts talking about the benefits of “free” — other large companies and any companies that depend on advertising are sure to pay attention. Then there’s Pinterest, the site that’s becoming a major feeder channel for retail. Members — about 60% of whom are women — post original pictures, photos of celebrity hairstyles, recipes, things they want to buy and more. Others comment or start their own threads based off the original post. The viral activity has caused traffic to soar, rising over 400% last fall to 7.2 million unique visitors by December. Where this curve ends is impossible to know: Does it rise to a Facebook-like magnitude? Drop off like MySpace? Is there someone in a garage somewhere getting ready to reveal a site that tries to out Pinterest. Pinterest? (No doubt, the answer is yes.) Or, what happens if there’s a seismic shift and the Internet landscape suddenly changes? As the BBC notes in its story on the G20 “Web economy,” even the current desktop world is just a few years from being completely transformed:

By 2016 about 80% of all internet users will access the web using a mobile phone.

That giants will rise and giants will fall in this new landscape is about the only thing that’s certain.

Here are the most shared stories by professionals in the following industries: