Facebook has notched up more 350m active users, but this latest milestone indicates that while membership continues to grow, it is beginning to slow down.
The last 50m users were added in 77 days, compared with only 62 days to jump from 250m to the 300m mark, according to reports.
This means Facebook, which reported its first positive cashflow in September, may struggle to sustain recent rapid growth as it plans to become a listed company.
At 350m, Facebook membership is just 100m short of the total number of worldwide broadband subscribers, which places a natural limit on its future growth prospects.
The price of Facebook stock jumped as much as 42% from July to November, with shares selling on private exchanges at around $21, putting the value of common shares at $9.5bn.
Facebook is widely expected to sell stock through an initial public offering (IPO) in the next 12 to 18 months.