Popular social networking sites are failing users by being "slow and inaccessible", a research firm claimed today.

Website monitoring company WatchMouse said that many web 2.0 sites are slow to open or fail to load properly. The firm monitored the time it took the social networking sites, listed on Wikipedia, to load.

The results showed that the worst for availability is the immensely popular Facebook, followed by Twitter, Last.fm, Windows Live Spaces, Friendster and Del.icio.us.

Of the 104 sites monitored, 51 showed a Site Performance Index (SPI) of 1,000 or more, making them very slow in load time.

"This is a remarkable outcome seeing as most sites use Ajax, which should lead to quicker load times since the dynamics of the site do not load immediately," WatchMouse stated.

"Using Ajax should help sites increase interactivity, speed, functionality and usability by exchanging small amounts of data with the server so the entire webpage does not need loading fully every time someone performs an action on a page."

Faceparty performed the best with an SPI of 303 meaning that users can access the site most frequently and in the fastest time.

But most of the sites still have a lot to do if they want users to keep returning. Research has shown that most web users are very impatient and will wait no longer than four seconds for a webpage to load.

"It is interesting to see that popular networking sites turn out to have very bad performance," said Mark Pors, chief technology officer at WatchMouse.

"It is surprising that they still have such a big fan base when they serve their users so badly.

"Using Ajax technology they should be able to work more effectively. For now the sites will need to do a lot of work to remain popular and improve their performance."