An existing post is an ad built from something you have already published on your Facebook Page or Instagram account, rather than a new ad created inside Ads Manager. Because the ad and the post are the same object, every like, comment and share from paid and organic viewers collects in one place.
How existing post ads work
Every organic post has an ID. When you set up an ad in Ads Manager, the ad creative step offers a choice between creating a new ad and using an existing post; picking the second lets you select a post from your Page or Instagram account, or paste its Post ID. The ad then shows the post exactly as published, with your chosen call-to-action button and website link where the format allows.
The quick Boost button on a post does something similar with fewer controls. Building the ad in Ads Manager instead gives you the full choice of campaign objective, audience, placements and tracking, while keeping the same post.
Agencies and media buyers also use Post IDs deliberately. They create an ad once, note its ID, and reuse that same ID in every new ad set or campaign, so engagement keeps building on a single post instead of being split across dozens of copies. Creator content can be run in a similar way through partnership ads, where the post stays on the creator’s account and both names appear.
Why it matters
Comments and reactions are visible social proof. A UK customer scrolling past an ad for a local gym or an online shop takes more notice of a post with dozens of genuine comments than one with none. Concentrating that engagement on one post makes it stronger over time, and a post that has already performed well organically is a reasonable signal that the message works.
It also saves work. A strong Reel or carousel from your Facebook Page can go into a campaign the same afternoon without re-uploading anything.
The UK advertising rules still apply. A post becomes an advert once you pay to promote it, so its claims must meet the CAP Code, and any creator post you promote must be clearly marked as an ad.
Common mistakes
- Promoting a post whose text was written for followers (“Big news, everyone!”) and makes no sense to a stranger.
- Duplicating ads instead of reusing the Post ID, which scatters engagement across many copies.
- Not moderating comments. An unanswered complaint sitting under a paid post is seen by everyone the ad reaches.
- Using the Boost button for sales goals and getting cheap reactions instead of enquiries or purchases.
- Planning to rewrite the ad later. The ad shows the post as published, so a new message usually means a new post rather than an edit.
How to act on it
Look through your last few months of organic posts and pick those that drew real comments or saves. Check that the opening line makes sense to someone who has never heard of you and that any claim can be backed up. Then build the ad in Ads Manager, choose the objective that matches your goal, and select the post as your creative.
When you find a winner, copy its Post ID and reuse it in new ad sets rather than rebuilding the ad. Set aside time each week to reply to comments and hide spam, and avoid any temptation towards engagement bait. Choosing which posts to promote and keeping them tidy is part of the Facebook and Instagram ads management I run.