Please see the ease of twitter button in your site.
New Twitter “Tweet Button”
For those of you who have been dreaming of an easier way for your readers to share your posts on Twitter, that day has come. We’re pleased to announce that we’ve added an official Tweet Button as an option for all WordPress.com blogs.
How it works: When one of your readers hits the Tweet Button, they will be shown a popup that includes a shortened link to your post. Readers can add in a quick message, and then hit “Tweet” to send the post to their Twitter feed as a tweet — all without leaving your blog.
Additionally, each time a reader tweets your post, you’ll know it: The tool keeps a live tally of tweets, so you’re never in the dark about how your blog posts are performing in the Twittersphere.
To enable the button on your blog please visit the “Appearance > Extras” menu and select the “Show a Twitter Tweet Button on my posts” option.