Sending API InvitesThe ability to send email invitations to other potential users is a pretty common tool in place in many applications, helping them hopefully to expand their user-base.
Phaxio, an API that allows developers to integrate faxing capabilities into their applications, has email invitation as a main menu option in its API area.
The Phaxio API is in private beta, but existing users can send off a Phaxio invitation code that allows other developers to discover and sign up for the faxing service.
Application email invites are clearly not something new. And with the growth of social media, it may be that sharing via Twitter or Facebook has become more associated with the steps necessary to make things "go viral." Nevertheless, having the ability to send out invitations via email can still provide essential word-of-mouth marketing for your API and shouldn't be overlooked.