The short answer: use the Boost button when your goal is engagement (likes, comments, shares) and you need something fast. Use Ads Manager when you're trying to drive conversions, sales, or any outcome that matters to your bottom line.
Meta's Boost Post button is optimized for one thing, engagement metrics. It shows your post to people who are likely to like, comment, or share it, not necessarily people who will buy from you. The algorithm prioritizes reach over intent [1]. It's like promoting in front of a crowd that gathers to socialize, not a crowd with money in their pocket. For low-ticket items (think $3-5 products), you might accidentally make a sale. For anything higher-value, you're mostly buying vanity metrics.
Ads Manager, on the other hand, lets you set a specific objective: brand awareness, consideration, or conversion [2]. It gives you audience targeting granular enough to find "working-class women between 25–40 who earn well," retargeting capabilities for people who visited your site but didn't buy, and the ability to track actual business outcomes through Meta Pixel and Conversions API. You're paying for intent, not just visibility.
Boost Post has limited customization, you pick a budget and audience size, Meta handles the rest as a black box [1]. Ads Manager gives you full control: campaign objective, audience layers, placement, bid strategy, conversion tracking, and creative testing.
One of the most common mistakes I see is teams using Boost Post without considering what they actually need to achieve. They get decent engagement numbers and think it's working, when they should be measuring sales or leads instead. That's a setup for disappointing returns.
If you're running multiple pages or boosting posts regularly, the friction of manually deciding what to boost and rebuilding the same audience targeting every time adds up fast. That's where rules-based automation makes sense, you set your engagement threshold and budget cap once, and proven organic posts get boosted automatically through real Ads Manager campaigns, giving you control without the daily busywork.
For the full breakdown of what each tool can and can't do, see "What targeting options does the native Boost button give you vs. Ads Manager?" and "Does the native Facebook Boost button use the same ad delivery system as Ads Manager campaigns?"