For starters, here is what the FlockDB readme says:
FlockDB is much simpler than other graph databases such as neo4j because it tries to solve fewer problems. It scales horizontally and is designed for on-line, low-latency, high throughput environments such as web-sites.
As one example, FlockDB isn't capable of doing deep graph traversal because it's designed to handle Twitter's single-depth followers/following model.
The video here has a pretty good summary from the CEO of Neo Technology where he briefly mentions FlockDB: http://www.nosqldatabases.com/main/2010/7/5/need-a-graph-database-look-no-farther-than-neo4j.html