As the number of people using a product increases, the core experience of any single user should get better.
It is especially true and necessary for social products. Every person added to your network increases the overall experience for others. If more people does not transcend to better experience for an individual, there are two things that are off. One, the individual has little incentive to ask others (mostly friends and family) to join them on this new service. What would they gain, if more people joined? Second, it shows stagnation of experience for the user. What's stopping them from moving to a new service tomorrow if it duplicates your features and does it better? This correlation between user growth and better experience of an average user is a function of design.
For Facebook, it means when more of your friends are on it, that's a better experience.
For Twitter, more people to follow and perspectives to what's happening.
For Quora, more questions being answered, more questions being asked, voted upon. All of this makes the individual's experience better.
For Dropbox, it gets easier to share files to anyone. And there is a reward to invite someone too.
For Google, it means you can share things like calendars with anyone on the Google system seamlessly. You can collaborate on documents together.
How does your product get better for the individual as more users join the service?
A product should also get better for the individual as they spend more time/effort into it.
A lot of this is the core of 'social'. It is no longer a feature, it is an expectation.
As a corollary, as more of my friends join a service, it should get better for me.