For instance their spam filter in gmail is almost flawless at least from my experience.
Anything that's about algorithms being part of the design they are great.
But they fail completely when it comes to allowing people to interact with each other. They simply don't have the design principles for that. It's not part of their DNA.
What Google doesn't do successfully, is frame those interactions to make their own network "sticky". It's those second-order interactions ("likes", "reblogging", "fans", "mayor", etc) that Google doesn't seem to get traction with.
But that's a very different thing than enabling interaction.
But the actual interaction part could be improved quite a lot.
Real people hated it because it was distractive, annoying and embarassing but they never tought about that because it showed their big technical achievement: the wavelet syncronization in real-time. REAL-TIME!!