I tried OSM yesterday and it couldn't find my friend's house address in the map. Not that it couldn't navigate there - it said the location didn't exist. Google maps finds it right away.

House-level addressing is probably OSM's greatest weak point, principally because house numbers are insanely boring for volunteers to survey. Try entering the street/town name without the number.

Did you add the address? The beauty of OSM is that anyone can edit the data set.

For some things it's much better than Google, hiking trails and bike paths for example.

I love OpenStreetMap for exactly this reason. For some time, Google gave me directions to an expressway ramp that had me driving around the block until, I suppose, enough people went forward where it told them they had to turn. Because it was across a one way street, and the lane had a that jogged slightly right (wrong way for the one way), it skipped it. But this would have been easy to fix on OSM yourself.

How do I add an address when I don't know where is the place? I can add this friend's address, because I've been to his house before. But what about a new friend?

Ask your friend to add it ;-)

But more seriously the idea is that if enough people start contributing to the map, there's no reason it can't be as good as Gmaps.

It's already quite good, depending on your area. In some places the government has supplied the data, which is part of the reason some cities or countries are very detailed, while others much less so.

Then you can’t, I guess. It’s not a perfect system.