Every single thing a user could do was based on FOAF-keyed permissions, and graph traversals were slow as hell. In implementing Facebook, Zuckerberg redefined

Every single thing a user could do was based on FOAF-keyed permissions, and graph traversals were slow as hell. In implementing Facebook, Zuckerberg redefined the problem space to give him an implementation advantage at every turn. Limiting it to manually-added colleges didn't just restrict the total userbase to be manageable -- it also introduced the concept of 'networks' that mostly eliminated the graph traversal problem. Photos were severely limited for a long time to constrain the storage problem. They also had a massive advantage in that they came of age in the post-bradfitz era of big consumer webapp architecture.