Nitish,
There is a hard-limit imposed by Facebook of only 50 friends that a request can be sent to. That's described on
this page
.
There is a way to pre-select or offer a "Select All" button. However, I haven't investigated how to do so because recently, Facebook changed their policies and will no longer allow pre-selection or a "Select All" button to be used after the end of this year. You can read more about that in
this blog post
. Specifically:
New Facebook policy, effective January 8, 2014: App requests can no longer offer a select all option or pre-select multiple recipients to receive a request.
Basically, Facebook wants users to intentionally think about each friend they are sending an invitation/request to instead of trying to get them to send them to each of their friends.
I hope that helps explain.
Thanks,
Alex