I know that there's a lot of banging on about privacy, or the lack thereof, on Facebook. However, there's one small thing that may have slipped your notice.
Your original 'default setting' was probably to publish your posts to 'friends only', which is sensible. (You'll find the 'default setting' under the 'cog' icon, along to the right of your name on the upper toolbar, under Privacy & Settings.) However: do you realise that if you change the privacy of an individual post, you then change that setting for the next time you use it?
Let's say that you've been happily publishing updates to 'friends', and then along comes one that you want to share to a wider audience - perhaps you want to advertise an event you're involved in and don't mind who sees it. More likely, you've 'shared' a public post - one of those 'thoughts for the day', charity appeals or attractive or funny images - which comes as 'public' to start with. When you share it, you keep that public status - and change your own default status to public, too.
It's very easy to tell what the status is that you're using. Look at your last post, and look at the icon at the right-hand end of the post. Here are a couple of mine.
I've got a cold right now (a particularly nasty one). Hence my silly post. It's shared with friends: that's the little two-person icon on the right, after the date and location of the posting. (Not that there's anything very security-sensitive about the fact that I'm feeling like death warmed up at the moment, but it's only really meant as a flippant remark for those on my friends list.)
This one's a bit different. It's a sharing of a video that I found funny. I have no problem at all with the post being seen by friends, friends-of-friends, or total strangers. So the privacy setting after the date is the little globe: public.
The point here is that after I'd shared, to a public audience, the silly Cockatiel video, my default setting for posts became public. So when I published my next update I needed to remember to change it back to Friends. Like this.
Why on earth you'd want to publish a post as 'only me' - in other words, nobody else could see it - I really can't imagine. However, the other settings all have their uses. 'Custom' gives you the choice to include or exclude specific people, and Public and Friends are what they say.
Have a check back on your Timeline and see what is posted as Public and what is shared with Friends. You can also view your Timeline as a non-friend would see it. Go back to the Privacy settings mentioned earlier, choose Timeline and Tagging Settings, and click on View As under 'Who can see things on my timeline?' It will display as a non-friend would see it. This gives you the chance to change anything that's not as it should be.
Going forward, it's easy enough to control as long as you keep your wits about you: every time you post, look for the little icon and make sure it says what you want it to say.