It is easier to blame targeted ads on something tangible like a conversation with a friend, without learning about the bigger picture. According to practices advertisers have admitted to using, it is more likely your previous transactions play more of a role than keywords in a conversation.
Let’s go back to our surround sound speaker situation:
While you are scrolling through your news feed, scratching your brain as to how Facebook would know of your need for speakers, try and think back to your recent purchases. Could it be you recently purchased your new tv at Best Buy? While doing so did you also hand over your email and phone number to receive reward points?
If so, it is possible a third party data collector compiled your purchase data along with others it receives from Best Buy. Then Samsung, the make of your new television, purchased this information and cross-referenced it with the tools Facebook makes available for its advertisers. Now they know where to reach you on your feed and can ask if you want fries with that shake ( or speakers with your TV in this case).
To avoid this: you can either avoid reward cards altogether or supply reward cards with an email not linked to your Facebook or any other accounts.
Additionally, Facebook uses several data broker websites that you can opt out of if you choose to. Note that these sites are notorious for making it difficult to do so.