If half of those internet users friends use facebook, this limits Sendrail to 1/4 of potential matches.
If half of those internet users' potential recipients of files are friends (colleagues, acquaintances, chat room contacts, etc may not be fb friends) that gets us to 1/8 of use cases as potentially solved with Sendrail, and Sendrail is only useful to Fb users 1/4 of the time.
Most people won't want to use a service that's useful only 1/4 of the time for a particular task, so this is far worse than a 50% reduction in potential users. This is how you get no users.
If half of those internet users friends use facebook, this limits Sendrail to 1/4 of potential matches.
If half of those internet users' potential recipients of files are friends (colleagues, acquaintances, chat room contacts, etc may not be fb friends) that gets us to 1/8 of use cases as potentially solved with Sendrail, and Sendrail is only useful to Fb users 1/4 of the time.
Most people won't want to use a service that's useful only 1/4 of the time for a particular task, so this is far worse than a 50% reduction in potential users. This is how you get no users.