Can i use hubspot fields to create a similar audience for ads? For example, i have a field in contacts where i say if the contact subsribed my services. Can i use that to direct the FB Ads to similar people?
You should be able to! I'm assuming that you are referring to lookalike audiences, correct? Since them subscribing to your services can be counted as a conversion event, you most certainly can create a customer list of all the people that have subscribed to your services. Once you have that list, upload that to FB as a custom audience, and then create a few LLA audiences from there.
Use those LLA as the targeting in your ads and you'll be sending your ads to people who are similiar.