Are you starting a social media company? Stop! Don't build a new social network, please. Just use Facebook. Yes, it sucks, but Facebook won. Or as a friend of mine starting a company recently told me, "we consider Facebook to be layer 7 of the Internet".
Apple just launched Ping, their iTunes social network. They also just launched Game Center, their gaming social network. These two social networks are brand new, you start with no friends. They're also disjoint. That tapping you hear this week is millions of Apple nerds adding their friends, again, to a new set of applications.
Apparently I'm an active user of 12 social networks: Facebook, Twitter, four instant messengers, Skype, Flickr, Metafilter, Blizzard Real ID, Steam, Xbox Live. That doesn't count the implicit social networks I've belonged to online for 10+ years, mostly via email. Then there's the social networks I no longer use or deliberately avoid, like Aardvark, Buzz, FourSquare, Friendster, LinkedIn, LiveJournal, MySpace, Orkut, ping.fm, Plaxo, Yelp, or Zephyr. And my actual social network, the people I call for a beer when I'm bored or whose kids I buy Girl Scout cookies from because that's how you do.
All of these social network applications just need one bit of vital information, "I know this person". And "person" is identified by email address. A list of acquaintances' email addresses is the universal social data.
I know, I know, your immediate thought is "but social media is more complicated! My users need to sort family from friends from coworkers and keep their nudie pictures private and what if someone changes their email address and how do I sell extra access to Zynga so they can spam my users' friends with credit card offers?" Yes, that's true, social relationships are complicated. Your special application can be as complex as you want, be assured you'll get a bunch of details wrong. But the core thing, the list of acquaintances' email addresses? Don't make a new one.
I'm a little glib up above by saying "use Facebook". Facebook puts a lot of conditions on using their social graph, enough that Apple won't use their data. What would be best for users and every Internet company except for Facebook would be a proper open standard for social data. Sadly OpenSocial is a failure and because of network effects I think it's too late to displace Facebook. So we're all kind of screwed.
(The saddest thing? I found my list of social networks by searching Gmail for the phrase "your friend". I found a lot of FourSquare spam but precious few actual friends.)
Update: quick clarification in response to feedback. I'm not saying "don't compete with Facebook", I'm saying "don't build something new where I have to enter a new list of my acquaintances."