The site racked up 2.1m unique visitors during October, an increase of more than 1.8m since Jupiter's last report in June, when Friends Reunited had 191,000 unique users.
Friends reunited has doubled its growth nearly every month while other sites in the same category - directory and resources - have seen user numbers remain static.
If the site continues to grow at this pace it could make it into the UK's top 10 most visited sites within months. The top 10 is currently dominated by long-established and well-known brands such as MSN, Yahoo!, Freeserve, Ask Jeeves and Microsoft.
The amount of time users are spending on the school reunion site is also growing. In June, when it first appeared in Jupiter's report, people were spending an average of 19 minutes a month on it.
This has since doubled to 41 minutes a month.
The site is said to be most popular among 25- to 35-year-olds and has more female visitors than male.
According to Jupiter's figures, 25% of UK women in the 25-34 age group and who used the internet from home visited the site in October, compared with 17.6% of men.
Lucy Green, Friends Reunited marketing manager, said, "With no promotional activity we have seen the site rapidly achieve a ranking among the top 20 sites in the UK for at-home internet users.
"The site has also been successful at attracting every age group, giving it special appeal to advertisers."