Depends on what you mean by 'walled off.' If Facebook/MySpace were both clients to a larger 'social network' network (think possibly USENET-style, or P2P-style), we wouldn't necessarily have youtube-esque comments showing up everywhere.
Social networks are mainly places for people socialize with friends. When people get onto youtube and start flaming people they are usually not directly flaming their friends (or at least not doing so from a username that their friends will recognize). Things like privacy settings, friend lists, etc are what help to regulate something like Facebook.
If there was a well-defined protocol for pulling/pushing just the information that was needed for a particular profile (say having all of my friends updates pushed to my account on whichever server it's being hosted), then it doesn't necessarily open up this decentralized social network to youtube-style garbage. For you to see the youtube-style garbage you would need to accept the troll onto your network. And even after they were exposed as a troll, you could banish them.
Social networks are mainly places for people socialize with friends. When people get onto youtube and start flaming people they are usually not directly flaming their friends (or at least not doing so from a username that their friends will recognize). Things like privacy settings, friend lists, etc are what help to regulate something like Facebook.
If there was a well-defined protocol for pulling/pushing just the information that was needed for a particular profile (say having all of my friends updates pushed to my account on whichever server it's being hosted), then it doesn't necessarily open up this decentralized social network to youtube-style garbage. For you to see the youtube-style garbage you would need to accept the troll onto your network. And even after they were exposed as a troll, you could banish them.