First, let's focus on what a Video View Impression is, as defined by the Interactive Advertising Bureau (IAB): 

A Mobile Video Ad that meets the criteria of 50% of the ad’s pixels on an in-focus browser or a fully downloaded, opened, initialized application, on the viewable space of the device can be counted as a Mobile Viewable Video Ad Impression if it meets the following time criterion.

Video Time Requirement: To qualify for counting as a Mobile Viewable Video Ad Impression, it is required that 2 continuous seconds of the video advertisement is played [...]. This required time is not necessarily the first 2 seconds of the video ad; any unduplicated content of the ad comprising 2 continuous seconds qualifies in this regard.

An important thing to remember when tackling this kind of count is whether you're talking about what you're paying for, or what you're reporting on. The two are different for Facebook. 

When creating video ads, you'll have to decide on what you'll be charged for (what you're paying for). For this case, we'll be focusing on payment and Optimization on Video Views. 

For now, Facebook has been counting impressions based on a 10 second viewing of the ad - these 10 seconds can be split into several sessions. What they're reporting on though is a 3 second view. 

Facebook is standardizing the way they count video views impressions:
To align with the IAB standards, they will now count as an impression every 2 second continuous view of 50% of the video's pixels. The reporting will also be aligned with this, meaning there will no longer be any discrepancy between both! 

You will now have the option to optimize for 2 second continuous view (the new IAB aligned option), or for 10 second views (potentially split). Both of them will then be reported on.
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3 Second video views are no longer going to be and option afterwards.

You can find these options in the Settings step of your campaign flow, in the Advanced part.

Here you select your optimization goal: 

The Video Views one is going to be the 10 second View option we mentioned above. 

Lower in the page, you'll select the "Pay for Each" option, meaning your billing event: