Social networking sites come and go. There hasn't been a social networking product that has really lasted. I believe it is because they were only a single product company and eventually people lose interest and move on to another product. Facebook, despite what they say, is also really a single product company. Everything they do is meant to get you to continue to use that single product whether it is with new features or apps (which are essentially features of FB that they got outside developers to build).
How can Facebook avoid the fate of other social networking sites? They only have to look to Yahoo. Facebook is not the next Microsoft, Apple or Google even though they're trying to be. The product and business offerings are just too different. What they are is really the next Yahoo. I think it's fortunate for them that their user demographic has changed to an older crowd similar to Yahoo's. If they didn't attract this crowd, they'd be like MySpace who lost the mindshare of their demographic and couldn't attract new users. The current Facebook/Yahoo crowd tend to not like change as much and are more conservative. At the same time, they also tend to be pretty loyal to what they know. While the media love to talk about the demise of Yahoo, the reality is that millions of users are used to going to Yahoo for news, sports, horoscopes and nothing will make them change their daily ritual.
This benefits Facebook even though there are short term pains as this group tends to be less open and more concerned about privacy so Facebook has to change itself to fulfill the needs of their current user base. Thus, I believe that Facebook has to decide if that is a future they want. Facebook likes to point out that they have surpassed Yahoo and they did it by becoming the next Yahoo. Now they just need to accept it.