"Your friend has sent you a message on our website, register to see it" - is there a name for this technique?

Question

Sometimes I get emails like the following, presumably targeted to me as a friend/contact of some person, X:

You have a message from X on our super awesome website! Click here to read this message.

where, if you click on the link, you are prompted to register an account on the super awesome website in order to read the "message" from X.

Is there a name for this practice of getting users to register by holding a piece of individually targeted content hostage?

Another example is the way some websites will tell you,

Five users have been searching for you on this site, register to find out who.

Answer 1

DarkPatterns.org refers to it as Friend Spam.

A site or game asks for your Twitter or email credentials (either via the password antipattern or via OAuth for an allegedly benign purpose e.g. finding friends who are already using that service), but then goes on to publish content or send out bulk messages using your account – i.e. from you. This technique is commonly used by viruses – but even well-known companies sometimes engage in “friend spam”.

Like other dark patterns on that site, it compromises trust and should be avoided.

Preventing access to the content once you click the link is another dark pattern called a Road Block.

When the user’s progress to task completion is restricted or stopped by something else on the screen.

Answer 2

Is there a name for this practice of getting users to register by holding a piece of individually targeted content hostage?

It's called a Registration Wall

This is an informal term coined for Marketers that is widely used on the internet. Just google the term "Registration Wall" and you come to loads of pages related to it but not a Wikipedia page.

This is similar to a Paywall: http://en.wikipedia.org/wiki/Paywall

Answer 3

I would think that a similar name for the practise would be a 'Referral Email', that is the mail was sent with the contact/friend's consent. It's becoming of a common practise for sites to ask the users to register with their social networks account and try to get the users to ask their friends to join in. This would be also a big part of new-new users acquisition.