1. Giant php codebase.
2. All the excitement is gone post-IPO, they've reached an inflection point and growth is slowing down.
3. Mark Zuckerburg doesn't have great social skills/intelligence. Which wouldn't be a problem, except that he runs a giant social network.
4. The corporate mission of making the world more open and connected just doesn't seem very high impact. Facebook at it's core is a way to share photos and keep up with old friends, it doesn't really change social interactions.
5. Their focus is very narrow, they don't work on many novel exciting problems.