If you know me personally, you are well aware of my disdain for Facebook and Twitter, yet my devotion to Google and all of its wonderful products and services. The majority of people who do not like Facebook or other social media outlets is due to "information sharing" or privacy issues. While I am not saying identity theft is not a concern, to these people I do think you should look at yourself in the mirror and realize... You're really not that important, and really, I doubt anyone outside of your social circle really cares about you in the least, unless of course you're someone famous.
While, I can see the value in using social media like Facebook and Twitter, to gain followers and really, free promotion/advertising, I still can't say I like it. I am likely coming off hypocritical seeing as how I have an account for each, but I rarely use either. It does frustrate me though, that now every single client I have seems to 'need' a Facebook page and 'Twitter' yet never want to update or set it up themselves. Worse is clients who think there is no point to having a webpage, and think they only need a Twitter or Facebook page. I for one, have never and will never go to a stores Facebook page before searching for their webpage. That said, if the company is of sufficient size (eg not a mom & pop shop) and doesn't have a webpage, I'll probably give up the search before looking for them on Facebook.
Now back to the original contention that the reason why people typically don't like these services is because of lack of privacy/selling of information. Most people feel strongly opposed to this, however, I think it is great!
Sit there, and scratch your head for a moment, but allow me to explain my reasoning. In life, by now, I am sure you have realized, nothing is free. Or most things at least have a price associated with them. With a lot of online resources, there is no price, because the cost is paid for by advertising. Now as much as advertising kind of sucks, at least you're getting what you want for free, so its a win-lose scenario.
Now that sounds ok, but, it gets better. Since these services typically know enough about you, they can and typically do, only show you with 'targetted' advertising, or things which are relevant to you. If I am going to look at an ad, I wont mind as much if it is at least something relevant to me, so its really not all that bad.
Final thoughts, although this wasn't covered, I don't like Facebook/Twitter, is because it never shows the 'real' person, it more shows who that person wants to be portrayed as. Typically I find I often loose respect for people after seeing their Facebook page. Some, not all, but enough for me to not like it. My Facebook wall has more or less become a glorified RSS feed with links to articles I have read of interest.
Lastly, if you read the middle section of the post, and still hate ads, do yourself a favor, and get Firefox/Chrome and install adblock plus as an extension. Or use Spybot Search and Destroy to block ads.