Ask HN: Are ad-blockers accounted for in # of "impressions"?

I have a small business and I sometimes advertise my business on social media. The cost of these ads is often based on "impressions" rather than click. "Impressions" means how many people saw the ad.

My question is, how does that work if people are using ad-blockers? If someone doesn't see the ad because they've blocked it, does that still count as an "Impression"? Do social media websites have a way to guarantee that an ad was actually seen rather than blocked?

I haven't seen anyone ask this question anywhere yet, and I think it's an important question with regards to the future of advertising on the internet. I also want to know if I'm wasting money.