First off, the 'find friends' area needs you to log in to your facebook, twitter or Google accounts. They ask you to give permission for them to use your WordPress account and this is exactly the same process as when someone logs in via twitter or Facebook to post a comment to ones blog. (Which personally I hate).
Here's a previous query about the Find Friends feature, and the response to it (not from me).
You should know that this is the way the internet is going, not just on WordPress.com. All sorts of services are linking up with each other and it's difficult to protect ones privacy.
As for keeping your blog's contents out of the possibility of public searches, if you've got anything that is personally identifying, from your cat's real name to photos of where you live, to a casual mention of something someone else can do a search on and find through a search engine, yes, your blog can be found. Anyone determined will find you - eventually.
Also bear in mind that while you've the option on your blog of not allowing google and other 'reputable' search engines to put your stuff out there, it won't stop disreputable ones. Oh - and these forums and their contents are open to the public and their contents can be found.
You should also know, if you want to protect your email address that if you use the comment form it shows the email address you've associated with your blog, to any blogger who sees the original comment (though the email address is not published in the blog itself). So if you get a stalker making a blog, then that's that.
Do change your email address to something non-recognisable, if you can. But bear in mind that then all your notifications and your subscriptions will go to that instead, so if it's one you don't usually use, you'll be missing a lot.
I'm not sure if I'll ever be able to do it, but I'm personally looking at moving to self-hosted where I won't have as many of these problems. Things can be overriden with plugins and while WordPress make the software for the blog system I want to use, they don't choose everything I want - and don't want - on my blog.
Have a look at this:
And bear in mind that WordPress.com - and Blogger and all the other free blog hosts are quite capable of - and do - change anything at the drop of a hat. WordPress.com in particular is always changing things. They even have it in their own 'about' page, which most people don't seem to notice before signing up or maybe they hope it doesn't mean what it looks like it means:
and this is the sentence to look for and believe:
"WordPress.com is under very active development, and we roll out updates almost every day."