Interesting: Author discusses getting invited to Orkut (first Google foray into social networking) in its early days and getting turned off when he started getting all these invites in Portuguese that were not relevant to him.
Theoretically, it is explained as such: anti-network effects occur when a community which has already achieved critical mass begins to lose value with each additional signup. The reason is that the core community that created the value to begin with starts to get marginalized and leaves
For example: In author's opinion, Quora is more “valuable” than Yahoo Answers. But I would argue that Quora could easily become Yahoo answers if, in the pursuit of “network effects”, they begin to dilute the quality of the community, and which would have the side effect of causing the most interesting and value-adding users to vacate. The Quora team is clearly aware of this risk, and are apparently steering the ship in such a way as to avoid this possible outcome.
Also of interest: the whole debate: go after the "mavens" or go after everybody?
The power of the asymmetric model + global feed
Twitter’s growth model is a nice blueprint for getting a critical mass and then growing to global scale. Specifically, if you launch with a small, dedicated group of interesting people that can asymmetrically follow each other, along with a global feed of all content posted, you can feel like you are the member of an interesting and vibrant community.
As the site starts to scale, the early userbase will depend less and less on the global feed, and use their own feed/following list to crank up or down the amount of information they are presented with.
The asymmetric follow model also takes care of some of the strange things that happen on Orkut, Facebook, Google+ etc. Strangers can choose to follow you, and @-reply to you, but it doesn’t feel like they are “putting” their troubling messages on your content.
It should also come as no surprise that Pinterest and Instagram followed the Twitter blueprint of asymmetric follows + global feed to scale from a small critical mass of interesting people into a massive, global community. Those sites were fun and useful to early adopters on a small userbase, and have managed to keep their community mostly solid throughout massive growth.