When you hit send on your email campaign, usually within minutes, that message is delivered to that contact's inbox. You are grabbing their attention in that moment.
Most social media platforms limit how many people see your post, and it is not delivered as soon as it is posted.
Email lives in that contact's inbox until they make a conscious effort to open it, ignore it, or delete it. Regardless, your message was seen, even if it was only the from name and the subject line.