Facebook's Huge Database
Strange as it sounds, graphs and graph databases are typically implemented as linked lists. As alluded to here, even the most popular/performant graph database out there (neo4j), is secretly using something akin to a doubly-linked list.

Representing a graph this way has a number of significant benefits, but also a few drawbacks. Firstly, representing a graph this way means that you can do edge-based insertions in near-constant time. Secondly, this means that traversing the graph can happen extremely quickly, if we're only looking to either step up or down a linked list.

The biggest drawback of this though comes from something sometimes called The Justin Bieber Effect, where nodes with a large number of connections tend to be extremely slow to evaluate. Imagine having to traverse a million semi-redundant links every time someone linked to Justin Bieber.

I know that the awesome folks over at Neo4j are working on the second problem, but I'm not sure how they're going about it, or how much success they've had.