What to do when your social network is no longer yours?

It was just over a year ago that Facebook opened up member restrictions to move from college kids to the rest of the world. This was clearly a good strategy as Facebook now has an astronomical valuation and a huge userbase. The question is what was/is the impact on the core audience (college kids) that made the site what it is today?

There are a number of sites that have sprung up like College Tonight and ConnectU, but none of them seem to be as captivating as Facebook itself. As TechCrunch's Mark Hendrickson writes:

Let's say you did want to capitalize on students' (growing?) discontent with the "mature" Facebook; what strategy would you follow? You'd probably want to take a few pages out of Facebook's own, er, book by restricting membership to users with .edu e-mail addresses, gradually opening up to elite schools, and keeping things stupidly simple. But you'd also have to provide something particularly unique, useful, or entertaining that tempts mainstream Facebook users to jump ship.

This is a common concept that growing companies (especially Web properties and open-source companies) have to keep in mind. How do you grow market share without isolating your core supporters?

For open source it means that you have to manage the community as you move toward commercial. For Web properties I would presume you have to make the features that hooked users in the first place remain compelling.

To some extent, this shows why SaaS applications like Salesforce.com and Netsuite are able to sustain for a much longer period of time--the users are isolated from each other, unlike MySpace.com, Orkut, etc., where you are potentially stuck with a bunch of people you have no desire to have a social relationship with...like college kids and their parents.