Who is your target audience? When I started writing my answer was, "Everyone who likes mysteries." That is a broad category. To be effective and to know your audience, you need to narrow that down a bit. You need data to see who your primary readers/followers are. And you can glean that information from your social media platforms. All of these bits of information will help you put together a picture of your readers and their likes.
How do you know who's interested in your type of book? You can use analytics and demographics from your followers.
Facebook - The amount of analytics you have access to on Facebook depends on the number of followers you have. Every time you reach a follower milestone, you'll be able to see more details. On your author page, click Insights. You will see information about page views and page likes. You can click on any of the information for more details like age, gender, and city/country of origin. This information helps you to target readers in certain age or gender groups.
Click on the Posts link to see how well your posts do. You can see the number of interactions, time of day when people visit your site, and which posts get the most/least attention. This will help you if you decide to do Facebook ads or boosts. This information helps you target the right groups and times for posts/ads.
Twitter - Twitter also has an Analytics link. There is a dashboard that will show you your top followers, top tweet, and number of follower mentions. If you click on the Follower Dashboard, you can see high level interests, occupations, buying styles, gender, and household income of your followers. It will also show you the top language. This gives you insight into who is following you and what type of tweets get the most attention.
Your Blog/Website - Check the analytics on your website. I get numbers on visits and comments. I also get yearly, monthly, weekly, or daily data about popular posts, who's visiting, what browser they use, and what country they are in. If you know when people visit your site, you can target posts on other platforms to drive traffic to your site. I get a lot of overnight visits when I'm sleeping. I use an app to schedule tweets in the early morning hours.
Goodreads - Use the information on your Goodreads author dashboard to view your followers. Each of your books has a wealth of statistics on the page. Also there is a link (far right) for See Top Shelves. This shows you who has saved your book and what keywords they used to find you. This is helpful if you're trying to build your keyword lists or to see how others view your book.
This is like an Easter egg hunt. Each site gives you different information that is often platform specific. But if you look at it holistically, you will get a better picture of your average reader. Mine is female between the ages of 35 and 65 from the United States, Canada, and Great Britain. It can also help when planning your marketing campaigns.
Best wishes with your writing and book marketing efforts. There are no hard and fast methods that work every single time, but your data will help you make decisions and provide you information on who's visiting your social media platforms.