“While each user has 130 friends on average, they have approximately 48000 friends-of-friends5. Even assuming only 4 bytes per posting list entry, this would still take over 178TB to store the posting lists for 1 billion users, which would require hundreds of machines.”— Facebook, research.fb.com
“Jon is also a friend-of-friends of himself since he is—by definition—a friend of his friends.”— Facebook, research.fb.com
“For storing per-entity metadata, Unicorn provides a forward index, which is simply a map of id to a blob that contains metadata for the id. The forward index for an index shard only contains entries for the ids that reside on that shard.”— Facebook, research.fb.com
“Unicorn also supports a Difference operator, which re-turns results from the first operand that are not present in the second operand. Continuing with the example above, we could find female friends of Jon Jones who are not friends of Lea Lin by using (difference (and friend:5 gender:1) friend:6). A…”— Facebook, research.fb.com
“Although there are many billions of nodes in the social graph, it is quite sparse: a typical node will have less than one thousand edges connecting it to other nodes. The average user has approximately 130 friends. The most popular pages and applications have tens of millions of edges, but these pag…”— Facebook, research.fb.com