Best Thing Since Sliced Bread!
Recently lots of internet marketers hopped on getting Facebook vanity URLs claiming them to be a second coming of domain names. But the problem with networks like Youtube, Facebook, MySpace, Twitter, etc etc etc is that you end up being someone else's user generated content, and it is virtually impossible to move a person from content consumer on those 3rd party sites into a customer on your site.
Social Media Traffic Typically Lacks Value
Most social media traffic is too fast and shallow to build a meaningful relationship from. Rarely are social media visitors worth as much as a penny.
Sure you can drop them at the top of a sales funnel, but then you still have to convert them. And the people who are ahead of the curve with technology are often the hardest to influence via advertising, and are the least receptive of offers unless they hear of them recommended from friends.
Social Media Mentions vs Independent Reviews
If someone recommends us, then I would much rather have that recommendation point at our site from their site rather than through a 3rd party website that might go away at some point. The third party recommendations on social networks tend to be brief/short/limited in context, so they don't carry a lot of weight toward selling something, and those mentions are often people whining about free stuff not being good enough and people recommending to their friends that they just grab a torrent of your work.

The natural bias of social media sites is toward people who value their time lowly (or else they would spend more of their time in tighter niche communities and/or in higher order business functions). Sure I have mentioned some of the recommendations for our stuff that people have done on social networks, but a link to a more in-depth review like this one is far more appealing because there is so much more context, and people who have read and followed that blogger for a while likely trust that review more than a random Twitter user trusts a 140 character recommendation.
Viral Does Not = Sales
Even the canonical example of proof of value of viral videos was not that successful. Millions of Will it Blend? video views helped the Blendtec company grow by less than a factor of 10. Many successful professional SEOs use SEO to increase the value of websites by that in less than a year, and have done so over and over again. With SEO you can create a million dollar business from scratch in about a year's time, largely because search has so much implied intent...so you don't need a huge traffic stream to monetize if you pick the right markets.
Domain Names vs Usernames
I was a bit slow to buy the seobook.org and seobook.net domain names, but recently bought the pair for less than $500. They allow me to further dominate brand related searches, while blocking potential competitors in the search results. For $500, that is not bad!
Some Facebook user named Peter Simik is squatting the facebook.com/seobook vanity URL and thinks I am stupid enough to pay $10,000 for it (assetize.com/accounts/view/210). I wouldn't give him $1 for it on principal. But there is not even a competitive threat there...people already have hundreds of connections to my real profile there, and few people are going to associate with the fake account.
More recently a couple readers highlighted that someone is on the newly launched Hunch.com website using my picture and our site logo to promote some crappy SEO website I have never heard of (hunch.com/seo-techniques/result/do-it-yourself/1928754/). That is obviously illegal, but it will only serve to undermine the trust in such 3rd party networks if they are full of fake & squatted accounts.
Protecting Your Brand
Services like KnowEm allow you to register your username on over 100 web2 sites to minimize any time wasting that might be created by someone hijacking your brand. Most of the web 2 sites will fail, but time is money, and it is hard to know which ones will be a success right from the start.
Even 1% of All Web Page Views Can be a Poor Business
Sites like Geocities , Anglefire, and Tripod were stars from about a decade ago, back when general web communities were hot then. And most of those types of sites failed.
A year ago MySpace was predicting great growth, but they have since rescinded on their big real estate deal, Google complained about how hard it was to monetize the traffic, and just earlier today the WSJ reported further job cuts:
MySpace announced plans to reduce its staff by nearly 30%, saying staffing levels were "bloated" and hurt its ability to be an efficient company. The social networking site, owned by News Corp., named a new leadership team earlier this year in an effort to reinvigorate the service. The move brings MySpace's U.S. work force to about 1,000 people.
As the Google ad deal is set to unwind MySpace, is downsizing their operation! According to Alexa, MySpace has over 1% of web's pageviews and they are struggling to make a business out of it.
How many of these general social media sites will be around in a decade?