Twitter is rolling out automatic link shortening on Twitter.com. Users can now simply paste a link of any length into the Tweet box on Twitter.com, and once you hit tweet, it will shorten the link so it only takes up 19 characters.

“Sharing links on Twitter.com is now simple and instant,” says Twitter’s Carolyn Penner. “Plus, since we show a shortened version of the original link, people will know which site the link points to. This service also increases security. If users click links that are reported as malicious, we direct them to a page that warns them.”

After 13 characters of a URL are entered, Twitter will show you a message letting you know that the link will be shortened. A link will be assigned a t.co link, but the link will still appear as a shortened version of the original URL.

T.co, Twitter’s link service, is only used on links posted on Twitter. URLs converted by Twitter’s link service are checked against a list of potentially dangerous sites to ensure safety.

Twitter will still let you use other URL shorteners, but you have to wonder how this will affect the other businesses that provide them. Twitter isn’t the only place people use URL shorteners, but it’s arguably the biggest reason such services started popping up so frequently to begin with. If you don’t have to shorten your URLs for Twitter anymore, how often are you going to use these?

Twitter has been very aggressive in its strategy this year, and third-party developers may be feeling the heat because of it. The company acquired popular client TweeDeck, which is sure to shake things up in the client competition area. Then they revealed that they’ll be letting users upload pictures, calling into question the future of services like TwitPic and YFrog. Now this.

The feature will only be available to a small percentage of users at first, but it will be coming to everyone soon.

If you’re interested in the effects of shortened URLs on SEO, read this.