These are some useful function for extending the jetpack sharing features.

jetpack_sharing_filters.php

// Add Twitter @username to Jetpack Sharing
// from http://wpspeak.com/adding-twitter-handle/
function custom_add_twitterhandle_via() {
    return 'username';
}
add_filter ( 'jetpack_sharing_twitter_via', 'custom_add_twitterhandle_via' );

// This function adds a colon after the title of the twitter share
function custom_sharing_title() {
        $post = get_post();
        if ( empty( $post ) ) {
                return;
        } else {
                // Create sharing title
                $sharing_title = get_the_title( $post->ID );

                return $sharing_title .':';
        }
}
add_filter( 'sharing_title', 'custom_sharing_title', 10, 3 );