It would be handy to be able to send a message to all your Facebook friends at once, but this isn't currently possible. Facebook limits messaging to 150 people and has no Select All feature to send messages. There are ways to communicate with all your friends at once, though: post on your profile, create an event, start a page and invite all your friends to like it, and create a group that includes all the people you want to interact with.
Message to All Facebook Friends
Sending a message on Facebook goes through the Facebook Messenger service. You can send individual messages or send to a group of people. To send a message to a group, you need to input each person individually. There is no Select All feature in Facebook Messenger.
Video of the Day
Even if you choose to input all your Facebook friends into the To box of Messenger, you run into another limitation. Facebook does not let you send a message to more than 150 people at one time. If you want to distribute information to everyone at once, post it on your profile. Be aware that Facebook users don't see every post on their newsfeeds, so you can't be confident everyone saw your message.
Create a Group Chat
If you want to send a message to all your Facebook friends at once, another option is to start a Group Chat. To do this, open the Compose Message box as you would when composing a personal message. Instead of New Message, choose New Group.
After the box pops up, input a name for your group and choose the people you want to include in the group. Moving forward, you can send new messages to everyone at once by choosing that group. Members can leave the group, at which point they don't receive notifications or future messages.
Create a Group for Friends
Although you can't send a message to all your Facebook friends, you can create a group and invite all your friends to join it. You use an extension to invite all your friends to your new group. Everyone won't accept, but you can communicate with those who do through the group.
It may be much easier to create a page and invite your friends to like it. If you've started a new business venture, this is an easy way to share messages. Go to your page and scroll down to Community on the left side. Click the Invite your friends to like [page name] button. Select See All Friends and choose Select All, then Send Invites. After they accept, check Send Message to send everyone a message.
Invite Friends to an Event
If you have something you're trying to promote, sell or convey, an event may be a better option. You can create an event and invite large groups of friends based on past events, locations and shared groups using a Select All feature. You can then host a live chat or post multiple messages over a designated period to reach your goal.
When you host an event and invite people, whether it's 10 or 1,000, only a percentage participate. You'll also inevitably annoy a few people by inviting them in the first place, especially if you're selling something. The best course of action is to emphasize how the event can benefit them and invite only those likely to be interested. If someone declines, don't invite them again if you want to avoid being blocked.
Facebook Messenger Etiquette
Technically being able to do something is one thing. How you do it is another. Even if you could compose a message and have it appear in the inboxes of all your friends, that may not be the best course of action. A Facebook Messenger survey found that 69 percent of people believe rules of etiquette should apply to digital communication.
If you're messaging all your friends to tell them about a product you're selling, your efforts may be seen as spam. This could lead to losing friends and possibly being blocked or reported. Facebook advises members to report unwanted spam, which could result in action against you. If you start a group chat and people leave the group, avoid reaching out to them in the future if you want to keep them as friends.
- Facebook Help: How Many People Can I Message at Once on Facebook?
- Facebook Help: Group Conversations
- Chrome Web Store: Group Invite All for FB
- Alejandro Rioja: Invite All Your Facebook Friends to Like a Page or Event on Facebook 
- Facebook Help: Create and Manage an Event
- Facebook Help: How Can I Block Facebook Event Invitations From a Specific Friend?
- Independent: ‘Messaging Etiquette Guide to Ghosting and Read Receipts Released by Debrett's
- Facebook: How Do I Deal With Spam on Facebook?