If you don't want to share with your friends, then don't.
But if you are a startup founder, remember the reason you see these viral sign up forms everywhere is because they work. You'd be silly not to offer the opportunity w/ your visitors because a small percentage will indeed share with their friends. Of course, that percentage is going down as people get fatigued or get burned by other startups that don't follow through.
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If you don't want to share with your friends, then don't.
But if you are a startup founder, remember the reason you see these viral sign up forms everywhere is because they work. You'd be silly not to offer the opportunity w/ your visitors because a small percentage will indeed share with their friends. Of course, that percentage is going down as people get fatigued or get burned by other startups that don't follow through.