I used to think that this browser is safe and does not track me. Then I did little experiment - i went to specific webpage on Opera and next day I found ads for it on Facebook app. How did it happen? Does Opera send my history for ad companies?
Every single browser tracks every single user! They need telemetry and analytics for a better understanding of the user's habits.
Actually, I think if you conduct the same experiment with a different browser, you will probably get the same result. Facebook, Google, and all the "others" have what it's called "activity trackers" which means that they can track you even if you're not using directly their services (typically, Facebook will track you even if you are not on Facebook).
Technically this could be done via the "like" button, for example, any website that offers a "share", "like" or whatever feature specific to Facebook, lets Facebook track you! More generally, every one (Facebook, Google,...) use also what's called "Web beacon" or "tracking pixel" which is:
An HTML code snippet that is loaded when a user visits a website or opens an email. It is useful for tracking user behavior and conversions. With a tracking pixel, advertisers can acquire data for online marketing, web analysis, or email marketing. With log file analysis, long data evaluation, or using appropriate analytical tools, this data can be used for different purposes, for example retargeting.
This said, and to answer your question, the fact that you were tracked, is probably more related to the web page you visited and not to the browser you used.