What Does Facebook Forced Invite Mean?
A Facebook forced invite occurs when a user attempts to install a Facebook application and is prompted to send the invite to all his or her Facebook friends in order to complete the installation. In 2008, Facebook updated its Facebook Platform Policy, and now requires that application developers provide Facebook users with the option of whether to invite friends to the application. As such, forced invites no longer occur on Facebook.
Techopedia Explains Facebook Forced Invite
Facebook was forced to place restrictions on application developers in 2008, following a rash of Facebook applications that were spread across Facebook by forced invites. Users were unimpressed by the bomardment of applications they received, particularly because many of them were of little real use. When Facebook announced its new policy, all application developers that were not in compliance were sent a notification; applications that still failed to comply could be removed from Facebook.