No, it's not paranoia... people on social media really might be seeing what you're thinking, to an extent. Any time you engage with any post; be it "Liking" it, "Loving" it, commenting on it or sharing it, there's a chance that just about anyone on your friends list, and even the general public online, could see it. It all has to do with the level of privacy both on your account, and the ones you're interacting with.
For example, on Facebook, if a post has a globe icon next to it, and you interact with that post, pretty much anyone typing your name in on a search engine could see that you did. It would also potentially appear directly in the feeds of your friends and family. Posts on Facebook that have a small icon of two people standing next to each other are "Friends Only", and would only be seen by anyone who's friends with both people. Posts that have a gear icon next to them are "Custom", and only the person who is posting knows who can view it, but interactions with such posts may still be shown to your friends and family.
As a matter of fact, you might be making all your posts unintentionally public, by accident. Be sure to take a look at your Facebook posts, to see if they have a globe icon (public), two people next to each other (friends only), three people next to each other (friends-of-friends), or a gear icon (custom), next to them. If you're posting publicly and don't want to continue doing so, see the Privacy settings on your Facebook account, and adjust them accordingly.
On Instagram and Twitter, any time you "Like" or comment on a photo on a public account, it too will be accessible for people on your friends list to see, again, potentially being shown in their feeds. The posts you interact with that all the people on your friends lists would not see, would be posts from private accounts on Instagram and Twitter. Only the people who are friends with those accounts would see them.
So, depending on the level of privacy you personally value, before interacting with a post on social media, take a moment to consider whether or not you want everyone on your friends list to see it - because, there's a better chance than not, that they will, and potentially, so will many other people that you don't know, too.