Jetpack’s contact form should need an option to redirect to a thank you page so that form conversions can be measured in popular analytics software like Google Analytics. Another option would be to fire a trackable event on the form submission so that an “event” is tracked in Google Analytics.

Here is a Jetpack add-on that adds such features:

• http://wordpress.org/plugins/42u-jetpack-booster/

Also suggested here:

• http://wordpress.org/support/topic/jetpack-contact-form-click-submit-and-redirect-to-thank-you-page?replies=3