I tend to agree with Danah about Orkut. In particular, I think Orkut does not model the real social network of an user. I speak of Orkut because is the buzz of the moment and its being in affiliation with google makes it the big expectation. But the same arguments could be used against many of the social network applications listed on socialsoftwareweblog.

The question is: “Why should I not accept an invitation from a totally unknown user that pretends to be my friend?” There is no negative consequence in adding someone as friend.

For instance, I pretended to be friend of Joi and Marc and Danah and Dina. And, even if they don’t know me, they added me to their friends list. I could be the worst spammer on earth but they didn’t care.
I did the same with people asking for an invitation in these comments. Because adding someone as friend does not have nay negative consequence.

Compare this with Epinions.com. In Epinions you can express your web of trust, a network of reviewers whose reviews and ratings you have consistently found to be valuable (from Epinions FAQ).
First of all, trust relationship is not symmetric. If I state that I trust Richard Stallman, this does not mean that he should trust me or needs to approve this. I’m only saying to the system that I’d like to see the items that Richard Stallman likes. [As a side point, it is worth note that trust relationship should have context. For instance, I can trust Richard for philosophy and free software projects but not for fashion or hair cut. This is not true, I’d love to have all his hair ;-)]
Moreover, your experience on Epinions.com is influenced by your web of trust. If you add “terrorist_user” to your web of trust, you will probably receive recommendations about bombs and flight simulator software but this is totally possible and acceptable. In this sense, you have incentives in keeping your friend list under control (this could also mean short) and in adding only really trusted and like-minded friends.

In the end of friendster, Dick reported how two guys in a shower room were competing about who had a larger social network on Friendster (“I’m now connected to 31,000 people”, at which point the second guy chuckled and said “Dude, you think that’s cool? I’m connected to 420,000 people”). Dick notes how the all point of becoming friends has become having as many “friends” as possible (or simply, more friends than you, that is the same). But being connected to everyone means being connected to noone!

So what are the possible solutions?
1) first define well if in your “community site” the relationship you want to model is symmetric (A states that B is her friend and B has to accept and confirm (or not) the invitation of A) or not symmetric (A can be friend of B while B is not friend of A; in this case the term “friend” is probably not semantically correct).
2) (especially if you want to model a symmetric social relationship) introduce some (possibly) negative consequences for adding someone in your friend list so that an user has the incentive to not add just every perfect unknown user and keeping her friend list under control.
3) limit the maximum number of friends you can have in your friend lists. I know this is kind of strange (“Who are you for telling me how many friends I could have?”) but, introducing scarcity, you are giving a value to friend acceptations. Instead, if the number of friends you can add is infinite, then there is no incentive in denying a perfect unknown the rank of friend. This has a lot to do with the economy of links and reputation economies (whuffie).
Maybe this number can be increased very slowly as long as an user keeps using the site but I’m not satisfact with this idea because I can create a bot that “uses the site” for me and lets me gain additional undeserved priviledges.
4) friendship could become a fine-grained relationship, i.e. you can add a value to your relationship, so that I can be friend of my mom as 1 and friend of my uncle as 0.6. In this case you can limit the total amount of friend currency you can spend, a sort of FriendShares (as in BlogShares.com?)
5) I think metaster () is something we really need to think about.
Perhaps we just need a web service for managing relationships on the social networking sites. A meta Friendster; Micrsoft Passport for social networking. We could call it, oh, I don’t know, Metaster…or Sterster. Sign in to all the sites with one username and password. Invite metafriends to all the sites with a single click. Manage a single profile across all the sites.
6) [what about you adding the 6th possible solution in the comments, my friend? ;-) ]
n+1) [of course there is always space for one more comment … and one more friend …]

I know it seems that I propose to monetize friendship but I only want to introduce incentives in having your friend list corresponding to reality.
Basically we need to break the assumption that having a large social network (even if not real) is better that having a small social network (but real). And also that having Joi Ito (or many power-bloggers) in your social network means you are more a blogroll-link deserver, i.e. a more interesting person. Summaryzing, if my relationship with Joi Ito is not real, Joi Ito should have some incentives in not adding me in his friend list.
Then you could ask: “what real means in our digital world?”
I have a perfect answer to this question but it doesn’t fit in the margin of this page. ;-)