Facebook, Inc. has lost 20.4 million US web users since the social network’s traffic peaked in November 2011, according to data from analytics company comScore. That’s a fall of 12.3 percent, or nearly one out of every eight users. (See source data.) Year over year, traffic fell by 17.9 million users, a decline of 11 percent.

Over the same 14-month period, social media rival Twitter has gained 4.2 million users, or 11.7 percent, while LinkedIn traffic has rocketed up 32.6 percent, adding 11.4 million users.

In regulatory filings, Facebook acknowledges its popularity has declined particularly among the young, writing that younger users are “actively engaging with other products and services similar to, or as a substitute for, Facebook.”

Facebook also admits to declines in web users both globally and “in key markets such as the United States,” declines it describes as “modest” if you look at global users and only compare quarter to quarter. The company additionally points out that its mobile traffic has increased, though it admits that it has “generated only a small portion of [its] revenue from the use of Facebook mobile products” since mobile advertising began. The company warns that without increases in mobile profitability, if users “increasingly access Facebook mobile products as a substitute for access through personal computers … our financial performance and ability to grow revenue would be negatively affected.”

Facebook’s stock closed at $27.45 Wednesday, a decline of 28 percent from its IPO price of $38 a share.