Well, not exactly. It doesn’t take away the “SEO” keyword benefit for two reasons:
1. Those links are nofollowed on Twitter anyway. Thus, no search engine benefit.
2. Truthfully, the SEO community has disagreed that there’s much SEO value to URLs at all.

Also, most of the time, your long URL will be truncated to a short URL regardless of whether you create the short URL or not.

Fortunately, some URL shortners (cli.gs does this for sure) give you the entire SEO value to links. That is, it implements a 301 redirect so that you can gain that “search equity” from using the service. Not all shortening sites do this.

“In other words, the long, contextual links tell more of the story than the obscure shortened link, both to the reader and the search engine.”

To follow the earlier point in tandem with the statement above, SEO != readability from a human standpoint. It’s important not to confuse the two.

It *does* take away the benefit for the user to determine what you’re talking about. But hopefully you’ll explain it in the other remaining characters and not in the URL.