Some advertisers may be questioning their investments in paid Facebook ads, but even the brands most unhappy with Facebook’s paid advertising offerings are, by and large, continuing to spend big bucks on their Facebook Pages.

Those Facebook Pages may not technically be owned media, but Facebook Pages are free, and brands have more control over them than anything else on Facebook, so they’re often treated like owned media.

But while Facebook Pages are free, acquiring fans isn’t and increasingly, brands are looking closely at acquisition costs. So it’s probably not good news then that two of the largest search engines — Google and Bing — are reportedly driving less ‘free’ traffic to Facebook Pages.

According to Facebook analytics firm PageLever, which looked at 500 Facebook Pages with at least 10,000 fans, Google referrals to Facebook Pages have dropped a whopping 51% year-over-year. Referrals from Bing are down even more — 59% year-over-year. All told, the largest search engine in the world is now responsible for less than 5% of external Facebook Page traffic, down from just under 10%.

The cause of this drop is not clear. At first, it appeared that the decrease in Google traffic may have been due to the company’s launch of Search, plus Your World, but that may not be the case. And any suggestion that Google was sending less traffic Facebook’s way due to its own social initiatives wouldn’t explain the drop in Bing traffic.

Whatever the cause, PageLever’s figures, if accurate, are probably a bit disconcerting to brands. The simple analysis: if organic search engine traffic is down, the cost of fan acquisition for a Facebook Page may, in theory, increase for some Page owners.

Of course, the fact that Google and Bing appear to be showing less love to Facebook isn’t all that surprising. Google, in particuar, has been particularly vocal about Facebook’s refusal to open its network up to Googlebot. Last month, Google founder Sergey Brin went so far as to refer to Facebook as one of the most “powerful forces” working against an open internet. “There’s a lot to be lost,” he said.

Whether Google is penalizing Facebook for not sharing more remains to be seen, but one thing is for sure: brands with Facebook Pages would probably prefer if Google and Facebook were friends.