comScore Media Metrix released its top 50 web properties for the month of June. It’s no surprise that Google sites remain on top with Yahoo and Microsoft in the second and third positions respectively.

Facebook has moved up 2 positions to number 6 though, giving the social network its highest ranking to date. Twitter actually just made its first appearance on the list, coming in at number 46 with 20.1 million visitors.

"It’s hard to believe that Twitter.com had fewer than a million visitors in June 2008, and now attracts more than 20 million visitors a month and ranks among the top 50 U.S. properties," says Jack Flanagan, executive vice president of comScore Media Metrix.

"Twitter has clearly been able to generate wide interest and cultivate a substantial following in a very short period of time," he adds. "If this momentum persists, then we will continue to see Twitter climb higher and higher in this ranking in the coming months."

Also worth mentioning is that travel sites jumped in June, although this is not incredibly surprising. "Travel sites typically see an increase in activity around this time of year as Americans look to get away," says Flanagan. "Summer vacation also means more leisure time for those out of school, leading to a seasonal increase in leisure categories including online gaming and entertainment."

Notice that career categories made gains as well. This is also unsurprising, considering the state of the economy and unemployment.

June was a great month for Google, according to new statistics from Nielsen.  In comparison to May, the search giant apparently received many more unique visitors, and also managed to keep them around for a much longer period of time.

How’s this for an increase: going from 131.3 million unique visitors to 147.8 million unique visitors in one month’s time.  That represents a gain of about 12.6 percent for Google.  Also, where time was concerned, it eked out an additional 32 or so minutes per person, moving from 1 hour and 17 minutes to roughly 1 hour and 49 minutes.

Of course, portraying an increase in time spent per person as a good thing can be misleading; the measure of a search engine’s success is generally how quickly it can send a searcher to another site.  But since more and more info is published on the first page of results, this statistic is part of a larger picture.

Anyway, Yahoo managed to keep the average searcher around for an extra 1.5 minutes in June compared to May, and attracted an additional 14.2 million unique visitors.

Bing, meanwhile, made its first appearance in this particular type of Nielsen table, and performed well enough by bringing in 9.9 million (or 9.7 percent) more unique visitors than Microsoft’s offerings did the month before.  The time spent per Bing/Microsoft user dropped by 18 minutes, too, bringing its data more in line with the industry leader’s.