A typical day for me pretty much relies on Google services. I start out by opening up my Google Chrome browser and checking my Gmail and Google Calendar. Then I might do a couple searches on Google Search. If I miss a call, I'll listen to the voicemail on Google Voice. I go to class and take notes in Google Docs. If I have time during my break, I'll punch out a quick blog post on blogger. Most days I use YouTube at least once too, to check newly posted videos, post my own video or just listen to a song. Also, during the day, to save minutes on my cellphone bill, I place calls through Google Voice on my computer. My Twitter also posts directly to Google Buzz as well.
Here's a quick known fact you may or may not have already known: both Blogger and YouTube are owned by Google. News to you? Don't feel bad, a lot of people probably don't realize that. Google probably owns more sites than we realize.
Also, all my pictures are managed by the desktop program, Picasa, which also syncs some of them online to the Picasa Web Albums.
So "they" know about every tweet, blog post and photo I put online, along with my schedule, who my friends are, what I watch, what I like, what I sound like, what I look like, what my number is, and where I live.
Did I miss anything? Oh yes... then you have Google Books that knows what you're reading; Google Finance that knows what companies you're interested in and potentially even your stock portfolio; and Google Feedburner and Analytics that track your blogs' subscribers and statistics. If you use Google Tasks, they also know what you want to accomplish every day. Don't forget about Google Location. If you have an Android phone, you most likely have Google Location enabled as well... so they probably know where you are right now.
Scare you yet? Probably so, but hopefully not. Why? Because if you use the web, you should have already been aware of these things. Personally, I use Google a lot as you can tell. They have all the tools I need! Some people are completely anti-Google, but I don't understand that. Google is just like any other company that has information about you. But does that mean you should be carefree? No. Absolutely not!
Quickly check out the video by Tinkernut, that talks about how to secure and manage your online presence without Google knowing. Much of this can be done through the Google Dashboard.
Bottom line, if you use any Google products, there's no real way to prevent them from knowing information about you, but there are some things you can do to prevent them from having excessive knowledge about you, like what websites you visit, etc.
Like he said, "...if you're not willing to understand it [the internet], then it's best not to mess around with it." Period. But wait, everyone uses the web and internet, we don't have a choice not to "mess around with it." Well, his point made exactly. Don't be paranoid and complain about privacy online. Instead, learn how to protect yourself the best way you can.