Emails are expected to be private. Publicly shared posts in a social network are expected to be public, so I would not be concerned that someone is reading them.
But they are not just doing this for publicly shared posts. Even posts which are shared privately will appear besides the ad when a person whom you have shared the post with is viewing the ad.
That by itself is not a problem. However, to curate the post, somebody from Google would have to read it. Else, they have really good natural language processors.
