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You didn’t provide any context for where you are making the call to xprofile_get_field_data.
Is it in a function? If so, what is that function hooked to?

Are you aware that there are 3 parameters ?
xprofile_get_field_data( $field, $user_id = 0, $multi_format = 'array' );

So perhaps your issue is related to the return of an array.
Try: $fb_page = xprofile_get_field_data('Facebook', $UserID, 'comma' );