There was a comment on Reddit a while back about what the 140 character limit means in different places:
In English, you get a comment.
In Japanese, you get a poem or a brief news article.
In Chinese, you get a short story... it might as well be a novel.
And someone gave an example Chinese tweet that translated to:
"You guys are the Sina fans other there? 'Keep on Perseverance' (some literate group) is publishing some online literature collection, referencing to the sissy poet character in the 'Cellphone' TV series. I know the people in 'Sina Literature Collection" (another group) well since junior high school; often went to their chat room to chit-chat bull. His (Keep on Perservance) main page's arrangement is overwhelming; I can't digest them. My preference is simple, slim, light-breeze style of literature. Hey, today is the clan chief's birthday, too. Don't know how he/she enjoying his/her day in Swissland. May be eating chocolate fondue? I can't do winter hibernation anymore. Have to come out. And have been messing around with you all day long."
There's a tradeoff between larger character set and easier input method. It's not trivial to input Chinese characters with keyboard. It compensates by allowing richer expressions with fewer characters.
Twitter gets millions of tweets and I am sure good percentage of them will have URLs. And if they start checking valid response for each URL, I think soon they will have to build a new data center just for this task. Also, what if the URL is temporarily down? What should they do?
I don't think they need to check for valid URLs. My point is just that disguising text as a URL will be possible regardless unless you check for 404s or ellipsize the URLs. Using a URL shortener isn't necessary to make URLs count for a constant character length.