In general, limit how much personal information you give out - just the essentials! - when:
Whenever possible, limit who can access your posts or your profile to just people you choose. (If need be, block users who don't respect your privacy.)
Whenever possible, limit who can save (capture, take a screenshot) your posts to just yourself.
Facebook has recently gotten attention for sharing users' personal information with business partners without notifying its users. This popular service has also had data security breaches in recent years. Both are good reasons to be careful about what you share, and to whom, on this app.
Facebook's default is to track your location, and use that information when you "check in" or post a photo. Better to turn off Facebook access to location in the settings of your device to block it.
The website Trusted Reviews has a more detailed article with directions on how to set different parts of Facebook to "Friends only" or "Private", and the Consumer Reports article linked below also has detailed advice for privacy settings.
Facebook's revenue comes from targeted advertisements, which use data about your posts and your "likes". Changing the settings for "Ad Preferences" (including the "Your categories" information) may limit the amount of personal information sold to advertisers. The Facebook Help Center page linked below describes how to do that.
Also check the recommendations about in this guide on posting videos & photographs, especially in light of Facebook's work on automated facial recognition.
Be wise about what you choose to share on these - and any - social media networking sites. Check the recommendations in this guide on posting videos & photos.
Companies are constantly revising their privacy settings and changing features, so check regularly to see what might have changed.
And do exercise your options to delete any problem posts or block problem users from accessing your posts.
Unless you change the settings, Instagram uses your location, and will also display your activity in real time to your followers, making it easy for others to track you.
Images shared on Instagram can be found by a Google (through third-party websites affiliated with Instagram) search unless you change the default settings to revoke third-party access.
In 2018 Instragram implemented settings that allow users to decide whether photos or videos can be viewed once only, can be replayed, or are saved to chat. Not allowing replay makes it less likely that someone else would capture your image or video (taking it out of your control).
Snapchat
Snapchat defaults to allowing anyone to contact you. Instead, change the settings so only Friends (people you choose) can do this. Also turn off See Me In Quick Add so that you don't allow friends of friends access to your information.
Likewise, change the settings so only Friends (or even better, only a Custom list among your Friends) can see each Snapchat story.
Pay attention to Notifications to see whether someone has taken a screenshot of any image or video you sent. (If they did, they could repost it anywhere, outside your control.) If you decide to save anything (Snapchat Memories), set it to My Eyes Only.
In default settings, Snapchat also records your location each time you use this app. Either turn off the ability to use location data at all times, or run this app in Ghost mode to avoid being tracked.
Since Twitter is a tool to broadcast your thoughts and comments to anyone who follows you, it may seem odd to think about privacy - but the Twitter company collects a lot of information about you and the people you connect to, in addition to whatever you write using that app.
In response to more protective privacy laws passed in 2018 for the European Union, Twitter recently updated limits how much personal data is collected -- but users have to choose those settings. The CNet article explains users' options.
Twitter settings also allow you to specify what advertising is pushed to you.