Maybe people got tired of their friends and family over the holidays, or perhaps they resolved to do a digital detox for the New Year. Either way, Facebook has been a bit quieter lately.

The number of Americans using Facebook fell by nearly 1.4 million in early December, according to new data from social media monitoring company SocialBakers. While Facebook FB, -3.48% has more than 167 million users in the U.S. and 1 billion worldwide, the recent drop in monthly active users is still akin to losing the entire population of San Antonio, Texas. “Facebook is possibly getting to a point where the less engaged part of the audience doesn’t visit every 30 days,” says SocialBakers CEO Jan Rezab.

Why the fall off? The increased advertising on the site and new experimental fees may be grating on some users’ nerves, experts say. Earlier this month, for example, Facebook tested charging users (with fees peaking at $100 within the U.S.) to send a message to someone outside their “friends” list. And in October, it rolled out the option to promote posts to more friends for a $7 fee. “There seems to be a change every other week,” says K. Jason Krafsky, who co-wrote the book “Facebook and Your Marriage.”

With a 54% share of the market in the U.S., some tech experts say Facebook’s growth was bound to start slowing. In fact, its market share may actually be closer to 80%, when you remove users under 13 years of age (not allowed by Facebook) and those over 65 (not big social networkers), Rezab says. “For Facebook, we don’t see a massive user growth opportunity driving the company’s financial results,” says Rick Summer, an analyst at Morningstar, so the company will continue pushing new features instead. (Facebook did not respond to requests for comment.)

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Of course, people come and go on Facebook, just as they gain and lose interest with anything in real life. “In the New Year, a number of people take a break from social, and others decide it’s a time suck, so remove their accounts all together,” says independent social media analyst Jennifer P. Brown. Brown says six people in her network quit Facebook in 2013 and she herself took a break from social media this month. Brown says she grew tired of the “vitriol” surrounding the current gun-control debate after the school shooting in Newtown, Conn.

“Students and teachers are headed back to school, and they might not need as much of their digital friendships now that they can interact more in person,” says Jason Keath, founder and CEO of SocialFresh.com, a social media training company. Others seem to have given up on Facebook completely. Brian Smith, a San Francisco-based environmental campaigner, for one, deleted his account, choosing instead to use Google+ for sports and international news, and Twitter for politics. “Facebook felt like a never-ending high school reunion,” he says.