It was a modest amount in the scheme of things — a $240 million investment by Microsoft in Facebook in 2007, valuing the social network at $15 billion at the time, and giving the Redmond company a 1.6 percent stake in Facebook.

More than four years later, Microsoft has held on to the investment. And it has already paid off strategically for Microsoft CEO Steve Ballmer by cementing the relationship between the companies.

Joint projects between Microsoft and Facebook have included advertising initiatives and a collaboration between Bing and Facebook to incorporate data from Facebook into search results — a step that Google is now attempting to follow by connecting Google+ to Google search.

Now, with Facebook poised to take its first steps toward an initial public offering, we’ll soon have a clearer sense for the financial value of the investment, as well.

Facebook’s value will ultimately depend on the assessment of the market, but the speculation for its debut generally ranges from $75 billion on the low end to as much as $100 billion.

That would translate into a five- to six-fold increase since 2007, easily boosting the value of Microsoft’s stake to more than $1 billion. Not a bad return, to say the least. (Facebook’s follow-on investment rounds aren’t believed to have significantly affected Microsoft’s percentage stake.)

Of course, the public market would also present Microsoft with a better opportunity to sell those shares. But a complete exit would be a surprise. With more than $51 billion in the bank, Microsoft doesn’t need the money.

And the real value of its relationship with Facebook remains that strategic link.

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