Facebook Pixel: A piece of code for your website that lets you measure, optimise and build audiences for your advertising campaigns. A code that you place on your website. It collects data that helps you track conversions from Facebook ads
Google Tag Manager: Quickly and easily update tags and code snippets on your website or mobile app. Tag Manager gives you the ability to add and update your own tags for conversion tracking, site analytics, remarketing, and more. There are nearly endless ways to track user behavior across your sites and apps, and the intuitive design lets you change tags whenever you want.
Facebook Pixel belongs to “General Analytics” category of the tech stack, while Google Tag Manager can be primarily classified under “Analytics Integrator”.
According to the community, Google Tag Manager has a broader approval, being mentioned in 543 company stacks & 203 developers stacks; compared to Facebook Pixel, which is listed in 24 company stacks and 8 developer stacks.