Wow, a tough question! I use them all, all the time. If I had to list them in order of daily-use, by time spent (and not necessarily by overall value), then I'd say:

1) Facebook - I use it multiple times per day, on the computer and my iPhone. I quit most of them, but was playing a few Zynga games on FB concurrently, so wasted many hours there. Not so much any more (down to just Mafia Wars again). Having it mobile adds to that use, especially with Geo-location apps (was using Gowalla, now Foursquare) posting to FB, and checking comments, etc -- it's part of my daily life.

2) Google - It's my go-to search engine, and the default in my browser and iPhone. It is the best search engine, and I use it often -- few times a day at least. I'm not sure if Google+ can overtake Facebook in social networking -- at least not for a long while -- and I don't use it yet -- but in the future who knows.

3) Wikipedia - As SV, Google points me to wiki articles upon a search. And I'm constantly looking things up when watching TV or movies ("who is that actor?", etc -- tho IMDb is better for that), talking with friends, or just in general. At least a couple times a day.

4) YouTube - Love it, but I'm not a daily user. But frequent enough.