1. How to save images
The Facebook app lets you download photos from your personal news feed - images posted by others, pictures you're tagged in, stuff like that. When you see a pic you want to save, tap on the three dots at the bottom and hit "Save photo". Photos are saved in a Folder named Facebook. Open your phone's gallery and it should be right there.
2. How to turn off chat heads
Chat heads are a handy feature to some, but an annoyance to others. Thankfully, they can be disabled from the Facebook Messenger app. Open Messenger, go to the third tab, and turn chat heads off.
3. How to attach images via Bing to chats
The Facebook Messenger app has this cool little feature that lets you attach photos of stuff to your texts - not photos you have on your phone, but photos pulled from Bing images. That should make explaining to others what a platypus looks like a bit easier. To attach an image from the web, hit the paperclip icon on the left, pick "Image Search", type what you're searching for, and pick a photo from the results by tapping on it.
4. How to hide unwanted posts or ads
If you have a friend who's posts you don't feel like following, here's a way of blacklisting them. Find one of their posts, tap on the little arrow in the upper right-hand corner, and choose "Unfollow". Don't worry, they will not disappear from your friends list. That's also how you can hide annoying ads from your feed. Should you ever decide to follow that blacklisted person again, go to their timeline, hit the "Friends" button under their name, and mark "Show in news feed".
5. How to (not) notify others that you've read their message
You might have noticed that the Messenger app displays a notification when a recipient sees a message you've sent. However, what you might not be aware of is that if you read a message in your notification bar or in chat heads without actually opening the Messenger app, the sender won't get a notification. If you want them to know you've read their text, make sure you open the app. Or don't open it if you feel like not letting them know you've read their message when you actually have.
6. Edit your favorites list
The Facebook app has a panel on the left where you can list items you access frequently - your photos, notes, friends list, events, and so on. Also, you can remove stuff you don't need. If you want to edit your favorites list, scroll down and find "Edit favorites" in the Settings menu. From there, you can add or remove items from the list. Also, you are free to move them around to your liking. Bonus hint: you can edit your favorite contacts as well. From the Facebook app, open up the panel on the right where your frequently contacted people are listed. There should be a big "Edit" button in the upper right-hand corner.
7. How to "dislike" stuff
Facebook doesn't really have a Dislike button, or at least it doesn't have one yet. You can, however, get a Dislike sticker. With a Messenger chat session open, hit the sticker button (the smiley), and then tap on the blue basket on the left. From there, find and download the "Likes" stickers listed under "Featured" or "All". Ta-da! You can now express your disapproval thanks to your newly acquired sticker set.
8. How to read details on messages
In case you ever need to know the specific time at which a message has been sent, hold down your finger on it until a menu pops up and select "Details". There you'll find the timestamp for that particular text.
9. How to delete chat messages
The Messenger app does let you delete messages you've sent and messages you've been sent. However, these disappear solely from your own chat history. In other words, the other party will still see whatever you've deleted.
10. How to leave group conversations
Being a part of a group chat can be annoying. Eventually, people will start chatting about stuff you're not really interested in, and the beeps you'll be getting while they chat can be distracting. Good thing there's a way of muting the group conversation, or leaving it for good. Open the Messenger app and then long-press on that particular group conversation. From there, you pick whether you want to mute or leave the chat.
11. How to (not) share your location
By default, Facebook Messenger will share your location with others. This can be disabled in case you find it annoying. Open the Messenger app, go to the Settings tab, and disable location reporting. Alternatively, you may open the Facebook app, open the panel on the left, scroll down to the Settings menu, pick App Settings, and disable Messenger location services. To turn locations on for a specific conversation, tap the arrow/crosshair before sending your message.
12. How to be a beta tester
Being a beta tester for the Facebook app has its perks. Most importantly, you will get the privilege of trying out new versions of the software before it is released to everyone. And you will be helping the Facebook team with making the mobile Facebook client and Messenger apps better. Interested? Read more on becoming a beta tester here