I read an interesting article the other day that theorizes that other intelligent, organic beings are likely to have been wiped out by the AI that they created. But the twist on that is that the AI wouldn't have done it knowingly, but in a similar way that we (humans) would destroy an Ant hill if it is on a construction site - in most cases the builders wouldn't actively 'want' to destroy the ant hill, and they may not even notice that they did it, but ultimately it's destroyed nonetheless.
It's a bit grim, but gives food for though.

Personally, I don't think we're alone. We are a race of babies (on the cosmic scale) on a tiny insignificant planet, in a tiny solar system in a galaxy that would be barely out of it's diapers. The universe is old, older than most of us can imagine, and far bigger than what our best technology can hope to see. In my opinion, it's impossible that we're alone.