People also spent significantly more time on Twitter, from an average of six minutes and 19 seconds in May 2008 to 17 minutes and 21 seconds last month.

However, compared with April 2009, Twitter's performance was more modest, as unique visitors increased 7 percent and average time per person actually dropping 1 percent, Nielsen Online said Friday.

Facebook remained at the top of the heap among social networks with 144.3 million unique visitors globally in May, its seventh month in a row as the leader in this category after dethroning MySpace.

Specifically in the U.S. Facebook also ranked first in May, its fifth straight month as number one, with 75.4 million unique visitors, an increase of 190 percent over May 2008, according to Nielsen Online.