When you boost a post, you can see Estimated people reached:

Estimated people reached is only an estimate and is based on several factors, including the average performance of all boosted post ads targeted to your selected audience.

Estimated reach is intended to help you figure out how much you want to spend on ads and isn't intended to be a prediction of how many people will actually see your ad once it's running. Competition for your selected audience, the size of your audience, the budget that you set and the quality of your creative may all play a role in how many people actually see your ad.

If there are more boosted Page posts running than estimated when you boosted your post, the price will increase and reach will decrease. You will only be charged for the actual reach of your ad.

Boosting a post that has been targeted to a specific location or language may also reduce your reach. The number of people that you reach by boosting your post is in addition to the reach that your post normally receives.

You can view insights for your boosted post when you go to your Page > click the Promotions tab.

You'll then be able to view a summary of all ads created from your Page, as well as manage, edit or pause ads from your Page.

* Nguồn: Facebook