From Facebook Developer Wiki

[edit] Test Accounts for App Developers

One of the hard things about developing on the Facebook Platform has been testing apps. Facebook's Terms of Service require that all accounts be linked to your real identity, so it's not allowed to make fake accounts to test things with. The status quo is the following cycle: developers make test accounts, developers test with those accounts for a little while, our Customer Support team comes across those test accounts and identifies them as fake and disables them, developers are annoyed and frustrated, developers make more test accounts. This is pretty bad.

So, we formalized the concept of test accounts in a way that should be better for everyone. Here's how it works.

To make a test account, register a new account on Facebook as you normally would. Become friends with your developer account and then go to this URL:
http://www.facebook.com/developers/become_test_account.php

This will mark your account as a test account by adding an affiliation to the Facebook Platform Developer Test Accounts network. As long as the account is affiliated with that network, it will be considered a test account. If you accidentally make a real account a test account, you can remove its test account status by removing the affiliation with the Facebook Platform Developer Test Accounts Network from the Network Settings page.

A few important things to note:

• Test accounts can access your applications when they are in sandbox mode only if they are listed as developers of your application.
• Test accounts won't be able to see real Facebook users and vice versa. You can make an existing account a test account, but if that account is friends with a real account, each user will disappear from the other's list of friends.
• Our customer support team won't disable test accounts for being fake, but test accounts may be disabled for violating any other reason that a real account would be disabled.
• Only real accounts should be used to administer and create applications. When a Facebook account becomes a test account:
  • That account can no longer use the Facebook Developer application.

Going forward, we'll add other functionality to the test environment – in particular, setting up accounts with large numbers of friend connections and making those available to users is something we hope to do shortly. But we're releasing this feature now to meet the immediate need of developers of a sanctioned way to test applications.

You can suggest ideas for ways to make test accounts better by submitting an enhancement idea into our public bug tracking system here:
http://bugs.developers.facebook.com/enter_bug.cgi?product=Facebook%20Tools%20for%20developers