With a 1.84% share of traffic, Twitter -- the category's hottest property -- is only a drop in the bucket compared to its larger rivals. But that proportion is up 1170% from 0.15% a year ago, according to the Web measurement firm.

Even so, Hitwise last month offered evidence that Twitter's torrid growth has cooled since April, when visits as a share of all U.S. Web traffic and searches on "Twitter" appeared to hit a wall. The proportion of new users going to Twitter from other sites was also down in mid-September compared to April.

Among social sites, Twitter's share dipped from 1.95% to 1.84% from August to September. While the company acknowledges that its figures don't reflect Twitter's mobile and application-driven traffic -- which probably accounts for the bulk of activity -- Hitwise says the Web figures should still correlate to new user adoption.

According to comScore, Twitter's monthly traffic has leveled off at about 20 million visitors since July, after jumping from 4 million in February to 17 million in April. The microblogging service could just be in a lull before another growth spurt.

Facebook, meanwhile, continues its steady surge, gaining three percentage points worth of share in the last month. The social networking site announced that it had passed the 300 million mark in users worldwide in September after hitting 250 million only two months earlier. While Facebook pushed past MySpace in traffic this spring, one area where MySpace continues to lead is in average time spent -- at 25 minutes, 56 seconds, according to Hitwise.

But even that edge is slipping away. Average time spent on Facebook increased 23% in the last year to 23 minutes, even as it dropped 12% on MySpace. It also increased 8% on social networking site Tagged.com to 25:17. Time spent on Twitter again showed a downward pattern for the service, as that figure fell 55% from 36:27 to 15:52. The sign of an addiction losing its grip?

Facebook has also shown strong growth among older users, with its share of those 55 and over more than doubling to 13.53%. Visits to social networking sites from that age group overall have increased 77% in the last year.

One demographic where Facebook has seen a steep drop-off, however, is among 35- to-45-year-olds, who have fallen from 30.3% of visitors to 18.4%.

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