IM (instant messaging) is an important part of the user acquisition process for many sites. Although many sites provide a way to invite new users via email, it is also important to take into consideration the capability to acquire new users via IM.

The Cool Link Test

A link passes the "Cool Link Test" if you think you're the first of your friends to see it and want to IM it to your friends and get a response of either "LOL," "Wow," "Damn," "Saw it," or "Heheheh." A link fails the test if it gets only "heheh," or "hmm", or "huh." I get multiple "cool links" every day from my friends via IM. I don't get any over email, except perhaps from my dad. Two characteristics stand out about IM vs. email that make it more appealing for sharing links. The first is that feedback from your friends is instantaneous. The second is that you don't have to remember anything to use it. You just copy, paste, click on your friend's IM name, and hit send. You don't have to remember their email address, or which email address they'll most likeley check.

Your Site and the Cool Link Test

Does anything on your site pass the cool link test? If there is nothing interesting enough, funny enough, or shocking enough to pass the Cool Link Test on your site, it will be difficult to acquire new users. If it is cumbersome to send a link via IM with immediately rewarding results due to registration requirements or links that break without the correct session data, user acquisition will be difficult.

Credibility

IMs have more credibility than emails. What happened in the following exchange is that my friend Jason invited his friend Joel to my site, and then had to IM joel to tell him to look at the invite. Jason told me that he had not customized the invite "because he is lazy."

jason:
i invited you to awesomesite.com
Joel:
whats that?

jason:
adam's new site

...

Joel:
if i saw this email ever, and you hadn't messaged me, trashed :)

You can allow your users to customize their invitation emails in order to make them less spam like, but there's a big chance that they won't use it. Email just has a lot less credibility than IM, so don't force your users to use email to let their friends see your site.