The limit is set by Facebook itself, and it's determined by a ton of factors. Mainly, the application that is trying to send them, the user that is sending them, how many have been sent from each, how well received they are, etc. It's all done to prevent spammy apps (or users) from abusing the system.
50 is the default number, from what we know, and that's actually the default for the user per day. So, if you send 50 from your app, you'd likely be unable to send any from other apps that day, unless that app has been granted looser restrictions by Facebook (which is not common from what we've seen).
Hope that explains. There's nothing we can do about the limits. While you may have thousands of friends you would want to invite to your site (for obvious promotion), most users likely won't be adversely affected by the 50 request limit.
Alex