New research from Hitwise has revealed a reversal of fortunes in the social networking world, with MySpace’s traffic decreasing dramatically, seeing its share tumble to 30.3% from 66.8% – a 55% plunge.

However, Facebook’s share of US traffic to social sites has grown nearly fourfold in the last year, to 58.6% from 19.9% as of September 2009.

Twitter, which has a 1.84% share of traffic, is only a drop in the bucket compared to its larger rivals, but its share is up 1170% from 0.15% a year ago.

Despite its meteoric rise, Hitwise indicated Twitter’s growth has cooled since April, when US visits as a share and searches appeared to hit a wall. It also pointed to evidence indicating the proportion of new users going to Twitter from other sites was also down.

According to comScore, Twitter’s monthly traffic has levelled off at about 20 million visitors since July, though concedes that the microblogging service could just be “in a lull before another growth spurt”.

Facebook recently announced that it had passed the 300 million mark in users worldwide, however, while it pushed past MySpace in traffic this spring, one area where MySpace continues to lead is in average time spent – which Hitwise indicates is an average of 25 minutes, 56 seconds.

Facebook has shown strong growth among older users, with its share of those 55 and over more than doubling to 13.53%, though one demographic where it has seen a drop-off is in 35- to-45-year-olds, who have fallen from 30.3% to 18.4%.