According to Nielsen, the number of unique visitors to Facebook rose 3.8% last month, reaching 135.7m.
Google was up by 3%, maintaining first place on 152.3m. Yahoo (131.3m), Microsoft sites including Windows Live and Bing (119.3m) and YouTube (105.2m) rounded out the top five.
Apple, in eighth place on 63m, was the top ten's biggest monthly riser, up 7.7%.
Across all sites, the amount of time the typical user spent on the web rose 5.8% for the month, from 55 hours and 22 minutes to 58 minutes and 36 minutes.
As well as closing the gap for first place in terms of unique visitor numbers, Facebook easily beat out its rivals for the amount of time users actually spent on its pages.
Facebook's average of six hours and 36 minutes was almost three times as long as the next-highest ranked company in the top ten, Yahoo. Google's total was one hour 22 minutes.
Nielsen noted that this latest set of month-on-month data is naturally skewed, as March was three days longer than February.
There was better news for Google in comScore's latest search statistics.
In March 2011, the firm increased its US market share, accounting for 65.7% of the core search market, up from 65.4% in February.
Yahoo sites were down from 16.1% to 15.7%, while Microsoft sites, including Bing, rose from 13.6% to 13.9%.
Data sourced from Nielsen/comScore; additional content by Warc staff