Once someone has engaged with your primary call to action it's a great time to get them to share your page with their friends. On average, having a "Share Page" option after someone signs up will increase your registrations by 30%.

It works even better if you offer incentive for people to share your landing page. For some tips here check out:

//www.kickofflabs.com/support/best_practices_for_the_viral_marketing_of_landing_pages

And

//blog.kickofflabs.com/the-second-conversion-most-people-overlook/

This share request can be made both on the landing page and in the thank you mail someone receives after signing up for your service.