Facebook: Be a supernode in the social network, and DO NOT attempt to own the network itself.
Google is a supernode in the web, on the XMPP network, and in the SMTP network. These are all open networks (which pre-date Google!) and yet Google is undoubtedly the biggest fish (or at least a large fish - thanks Larry for the correction) in these ponds.
In fact, Google embraces the philosophy of being "a big fish in a big pond" to the core. While they are one of the largest providers of web search, XMPP chat, and web mail, they do not erect artificial barriers of interoperability or user mobility. It is so successful because it supports an ecosystem of users, content, and applications that extends way beyond its corporate borders and embraces competitors as a vital part of that ecosystem (do you ever have any problems sending email from gmail to yahoo email?)
Facebook, you are creating an environment where users can't leave without losing that which they invested in (the part of the social network/web that is centered on them). Give them the choice and ability to leave and interoperate with competitors. Then, these users will have more confidence in you, knowing that they are not forced to make a choice between a) accepting whatever you force upon them on the one hand, or b) losing their investment in digitizing their social network. This availability of user choice would constrain your behavior and make you compete for their "social web hosting". In turn, you'll be forced to provide better services, better experience, and will continue to grow - presumably you'll maintain your lead if you are truly delivering value for users. You'll be a supernode in a much larger "open social graph/network" that, in absolute terms, will be far larger than any network you build walls around.
Its really simple: better to be the biggest fish in a big pond rather than a big fish in a small pond..