I go on many websites and enjoy them but, I wouldn't have even found or known about a majority of them without Google. Google will forever be my home-page! And it's a great spell-check tool, too. And Dictionary. And Thesaurus...
When not on Skype I use Facebook all the time to collaborate with my project partners for school stuff and to just see what is going on in my little world, so much easier than having to text/call or even go out and meet up with people to talk about these things, Facebook's chat service shines when it's really late and I need to talk with people. Google serves a more educational purpose though, I use it all the time to do school research, translate random things and occasionally as we all do just search random stuff and see what comes up.
Probably Google, or in broader terms, search engines (although I hate Bing and Yahoo as search engines). And if we want to get more specific, Wikipedia.
The number times I use my phone/tablet to look up information in school or on my couch is insane. Naturally I love learning, consequently it drives me nuts when I can't remember something I learned in school that day or a week ago (and unfortunately I forget a lot :/). So without fail I will immediately type in what I'm trying to remember or figure in to Google, which 99% of the time takes me to Wikipedia which is a valid source because it's always right despite what my teachers say so screw them I don't even care no more, and BAM I have what I'm looking for. Without search engines, I'd have to collect a vast library of knowledge, even more vast than that library in the desert in Avatar: The Last Airbender, to quench my appetite of remembering everything. And not only would I have to collect it, but I would have to sort through an incomprehensible amount of pages just to get one tiny piece of information that I'm trying to remember. And like I said, i forget a lot, so that would be time consuming and doesn't sound like a lot of fun.
I really like Twitter except when there's always that one person who tweets like 5+ times a day and shares everything that's happening as if anyone cares.
I like Tumblr because people keep their mouth shut for the most part. I'm still kind of disturbed at how they did the right thing by removing Thinspo but then turned around and okay'd Fitspo when it's really just Thinspo in disguise MOST of the time.
Kinda liked Google+ there for a minute because it had potential to be non-bookfaec, but now it's become a horrific chimera... a cross between Tumblr and Facebook, and now the NEWEST change makes it look kinda like Pinterest. Google has a serious identity crisis.
That's it as far as social networks go. Honestly, I could live without them. However, I think the most important internet feature to me is news. It's available RIGHT NOW. Hear a car accident outside? There will be a news report about it on the internet in 5 minutes. Storm coming, road work, etc.? Info available immediately. All that important stuff. I could easily live without it, but I think it's really made a difference in how I plan out my schedule, get places, etc.
My top three would be DGN, YouTube, and Google. I go on each of these sites multiple times throughout the day. I could probably find a way to manage living without them, though it would take a lot of effort.