Micro-blogging service Twitter, often criticised for its frequent system outages, got its act together in the second half of 2008.

Although Twitter ended the year with an embarrassing 84 hours of downtime, it showed significant improvement starting in July, according to a study from web monitoring company Pingdom.

Twitter recorded 84 percent of its downtime in the first half of the year and finished 2008 with uptime of 99.04 percent, the worst among the 15 social-networking services included in the report.

"July and onward has seen a big improvement in site availability for Twitter," reads the report, titled 'Social network downtime in 2008'.

Facebook, MySpace, Classmates.com and Xanga all had uptime above 99.9 percent, as well as Imeem, although the latter was monitored starting in May, Pingdom said.

Rounding out the top 10 were Google's Orkut (11.2 hours), Last.fm (12.5 hours), LiveJournal (16.1 hours), Microsoft's Windows Live Spaces (17 hours) and Hi5 (22.1 hours).

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AOL's Bebo ranked in 11th place with 38.6 hours of downtime, followed by Reunion.com (41.9 hours), Friendster (43.8 hours), LinkedIn (45.8 hours) and, in last place, Twitter.

Reunion.com had the longest continuous outage among the group with an incident that kept it offline for almost 10 hours in late March, Pingdom said.

LinkedIn, a site for professional networking, saw its downtime steadily worsen as the year progressed, according to the report.

Pingdom tested each site's home page from multiple locations in Europe and North America every five minutes. It flagged a site as being down if, from at least two of its locations, it couldn't reach the site, the home page wouldn't load within 30 seconds or it displayed an error message.