If your goal is X million users or some such end-user metric, you have created a goal for your salesmen, not your engineers.
An engineer cannot sign up a user, except indirectly through quality work. "X million users" is only indirectly under the control of the engineer: intermediates like sales or getting-the-word out are out of the engineer's control and can be screwed up by others. If the engineer does not get to pick his own features to work on, then the work he does do is limited by the wisdom of the person making assignments.
In other word, a goal like this is discouraging for an engineer because he lacks serious control over whether or not the goal is accomplished. It is an anti-motivator.
Instead, goals for engineers should be things directly under their control. Are you assigning specific features to the engineer? Getting a list of them done with high-quality solutions can be a goal since that is entirely under the engineer's control. Want to get your low-cost solution adopted? Set a goal for the engineer to reduce memory space by Z percent or improve performance by Y percent.
Always make sure goal achievement is controlled as much as possible by the person receiving the goal.