Micro-blogging Web site Twitter said it was defending itself from a denial-of-service attack Thursday.

The site has been down for several hours.

A denial-of-service attack is comprised of an overwhelming number of requests for the Web site. It works by overwhelming the servers with erroneous page requests, thus making it impossible for the server to function properly.

Way back when, Twitter outages were so commonplace that it was worth reporting when it didn’t crash-as when it stayed afloat during the entire South by Southwest Interactive Festival in 2008. Now, a few million dollars of venture capital later, the service is far more stable.

This is problematic, because Twitter wants to establish itself as a communications standard rather than just a social-media brand. It’s been a crucial platform for information exchange in the face of global events where more traditional means of broadcasting have been inaccessible or blocked.

Facebook also seemed to be experiencing problems, at least intermittently. Facebook spokeswoman Brandee Barker said the company was looking into it and would have an update “as soon as possible.”

The Twitter outage began at about 9 a.m. EDT, said Ken Godskind, chief strategy officer at Web performance monitoring company AlertSite. The site still had access problems late morning.

(Source: CNET / WCBSTV)