This is no longer necessary now most phones can send mails and display full web pages, but the limit has remained in place because the immediacy it created is almost addictive for many people. While Facebook now allows posts as long as a novel, tweets have to be short and to the point, so they take little effort to read and write (although writing good tweets is a skill in itself). This obviously gives companies little scope to tweet details of their products and services, but the common practice of simply tweeting a link gets around this while still taking advantage of the speed, ease and vast audience Twitter provides.