Cut the free atoms to 60. Charge $1 more for Calendar or Google apps integration. Charge $3 more for SMS notification/forwarding, allow them to post from their phones. Add atom sharing and give it away for free. Add a mechanism to allow people to make their atoms public, or invite others into edit/ownership roles.

Anything that adds convenience for 1 user you should charge for. Anything that invites others to use/consume the service, you should give away for free.

Don't forget the footer in atoms to say "Created with Atombox; make your own atoms aye href equals quote http://atombox.com quote AtomBox end-aye.