Twitter: Missing multiple "account" (identity/stream) management. SMS based string limitation.
Buzz: Missing identity/pub stream management, overpowering ties to critical Google identity (heavy baggage, gmail, etc). Public profile by design. Lazy "if it sticks" design. Cross-Google coherence problem (Reader notes, Orkut, etc).
Facebook: Malign business model. Zuckerberg.Of course there are other alternatives to consider. There's AOL, Yahoo!, Microsoft, Myspace, the Wall Street Journal, IBM, Oracle, Walmart, Newsweek ... Right. All equally irrelevant.
The big three are, amazingly, missing the target.
There's room for one more.
What would Apple do?