Would-be LinkedIn IPO investors, take heed: you might not be buying what you think you’re buying.

The professional networking service, like Facebook and Google years before, has a dual-class stock structure in place to ensure that co-founder Reid Hoffman and LinkedIn’s early venture capital supporters control voting power, even post-IPO.

Class B shares, owned by Hoffman and VCs, will be worth 10 votes each and altogether make up 99.1 percent of voting authority, according to SEC documents. After the offering, Hoffman will himself own 21.7 percent of the total vote, Sequoia Capital will own 19.3 percent, Greylock Partners will own 16.1 percent, and Bessemer Venture Partners will own 5.2 percent.

The remaining 37 percent or so of Class B stock is shared amongst smaller stakeholders, which includes other investors and company employees.

Anyone who purchases from the 7.84 million shares being offered tomorrow in LinkedIn’s IPO will be buying Class A stock, which altogether holds less than one percent of the Class B voting authority.

And that’s all without even considering the (perhaps not so good) financial prospects of buying shares in LinkedIn’s public offering.

Now, bear with me as I bring Facebook into this story for the umpteenth time.

There is little doubt that, even for those not a tad interested in purchasing LinkedIn shares, the IPO has attracted boatloads of investors curious to see just how the first-ever public offering for a U.S.-based social networking service plays out. Among other things, it will give us a glimpse into what a Facebook IPO could look like, and everyone wants to know how that will look, even if it is still at least a year away.

Like LinkedIn, however, Facebook--back in November 2009--created a dual-class stock structure, assuring that CEO Mark Zuckerberg and other large shareholders maintain control of the company.

Either way, LinkedIn is debuting on the New York Stock Exchange (NYSE) on Thursday under the ticker symbol “LNKD,” and shares have been hiked from the original $32-$35 price range up to $42-$45. In turn, the company’s valuation has been boosted from $3 billion to $4 billion.

I guess the underwriters think people are really excited about social media. (Well, aren’t we?)