I'm thinking about developing a viral invite type function to allow users to invite some of their contacts from popular OAuth services like gmail, facebook, twitter, linkedin, etc. Before I go there, though, is there any research on how often users use these functions if they are around?
I've never used such a feature, nor have I seen any data one how often such a feature is used.
But I do have a strong hunch that in many (and possibly the majority of) cases when the address-book inviter is enacted, the user does not have full awareness of its implications, and is often pushed into it somewhat coercively. Developers often prey on less-sophisticated users with these features.
I know that I have been rather annoyed in the past by invitations to obscure web services sent to me by obscure former contacts. I know that I would be mortified were I to somehow unintentionally spam my address book. And I know that I am quite turned off by websites that aggressively promote such features.
Bearing this in mind, recognize that such a feature could lend your service a certain air of sleaziness. Thus, if you are certain that your service is of such high appeal that your users would affirmatively desire to invite their contacts by automated means, implementing this feature in a fully transparent and explicitly optional manner is crucially important.
The obnoxiousness of the inviter can be somewhat ameliorated with clever implementation, such as allowing the user to selectively invite only certain contacts. But a machine-generated invitation will always be inherently spammier than the personal email that would constitute a true recommendation of a service.