Yes you can!
Originally, we opted to use http:// protocol by default because it resolves more quickly. Verifying an SSL ceritifcate for each stop in a redirect can add up to 100-400ms to our link's resolution, which usually doubles the time to resolve. We worked very hard to keep this number as low as possible, hence the http:// default.
However, after enough feedback from our clients, we decided to provide https:// as the default for all the links built in our dashboard, which is now the standard.
If link resolution speed is important to you, you can manually remove the 's' from any https://geni.us/ link when you publish it and you will have yourself a link that resolves nearly twice as fast!
Note: According to our research, when you use our domain to redirect links, the lack of an SSL certificate on the link itself does not impact your website's SEO or pagerank.