Facebook may be nearing 700 million users with a $100-billion IPO on the horizon, but all may not be well in Zuckerberg land. According to Inside Facebook, the social networking giant lost nearly 6 million users in the United States in May, along with 1.52 million in Canada and hundreds of thousands in the United Kingdom, Norway, and Russia. The company still managed to add 11.8 million new users worldwide last month, but its growth has been slowing significantly. What's happening? Here, four theories:

1. Facebook has made as many friends as it can

Mark Zuckerberg and Co. may be "hitting a saturation point in key markets," especially in the U.S., where roughly 50 percent of the population is already on Facebook, says Kent Bernhard Jr. at Portfolio. If that's the case, the social network might not be able to reach Zuckerberg's goal of 1 billion users without conquering China (and its strict online censorship).

2. People are sick of Facebook

"I think users are deleting their accounts because they... are burnt out," says Lindsay Mannering at The Stir. Even Bill Gates, a Zuckerberg friend and Facebook investor, recently quit the social networking site, saying his friend requests had gotten "out of hand." I don't blame him. "Between the feeds and the friends, it's too much... more of an obligation than a fun way to pass a few minutes." No wonder people are logging off for good.

3. This is just a temporary dip

"Seasonal changes like college graduations, and other short-term factors, can influence numbers month to month and obscure what's really happening," says Eric Eldon at Inside Facebook. These May figures are certainly intriguing, but let's not overreact. The long-term trends are the ones that really matter.

4. Other social networks are on the rise

It's "worth noting" that Twitter and LinkedIn are gaining in many of the areas that Facebook saw big losses — namely the U.S. Canada, and the United Kingdom, says Robin Wauters at TechCrunch. But let's not forget that "on a global level... Facebook is drawing more visitors than ever."