Twitter, which shut millions of users from using the online messaging service on Thursday morning, has come back online. However, efforts to defend against and investigate the attack continue, according to the company's blog.

Twitter post a message in its blog this morning as follows:

Twitter is the target of a denial of service attack. Attacks such as this are malicious efforts orchestrated to disrupt and make unavailable services such as online banks, credit card payment gateways, and in this case, Twitter for intended customers or users.

This is the first major outage of social-networking service; all Twitter users around the world were affected.

Denial of service attacks swamp a Web server with false requests for information, overwhelming its normal operations and eventually crashing it.

Recent attempts to access Twitter have had varying degrees of success, with long load times and occasional network error messages.

Twitter, which enables people to micro-blog in 140 character spurts, has exploded in recent months - its 44.5 million unique visitors in June 2009 represents a 1,460% increase in traffic from June of the previous year.