There once was a time when authors could write their book, hand it to their agent, and let the publisher do the work of  writing, editing, publishing, distributing, and marketing it.

For most authors, those days are long gone. Authors today are turning to self-publishing to get their work into the hands of readers. 

In addition to wearing their writer's hat, self-published authors need to do just about everything that goes into packaging, marketing, and selling books, like building an author platform. 

And one of the most powerful ways to build an author platform today is through social media. Social media lets authors engage directly with readers and build a tribe of loyal fans from just about every country on earth. 

Here are 10 social media strategies successful authors use to engage with readers?--?and sell more books:

1. Post user-submitted photos on your blog

Mark Vanhoenacker, a 747 pilot for British Airways and author of the critically acclaimed book about modern flight, Skyfaring: A Journey with a Pilot, invites readers to submit photos of the view outside the window of their plane, which he posts on his blog

2. Post photos of your travels on Instagram and Flickr

New York Times bestselling thriller writer Joanna Penn loves to travel, and seeks inspiration and conducts on-the-ground research for her novels during her globetrotting trips. She posts her photos on Instagram and Flickr, giving her readers a front row seat as she explores the world.

3. Produce Youtube videos of yourself as you write your next book

In March, thriller novelist Chris Fox produced a daily Youtube video in which he showed the exact process he followed to write and publish a complete novel in just 21 days. At the end of the three-week challenge, Chris successfully completed his novel and uploaded it to Amazon, where its Kindle sales rankings quickly rose, fueled by an appeal he made to his email subscribers.

4. Create a Facebook community around the theme of your book 

Blogger and bestselling author Jeff Goins created a Facebook community around the themes he features in his book, The Art of Work. In addition to sharing stories about his work and family life, he invites community members to share their stories of how they've discovered their calling in life, and encourages them to celebrate their successes. 

5. Crowdsource your book titles

Steve Scott, a top-selling indie author of self-help books on Amazon, often turns to Facebook communities to crowdsource his next book titles. He runs polls with options for book titles, and invites community members to weigh in with their votes and comments. 

6. Give advice on Twitter

Popular blogger and bestselling author James Altucher hosts regular Twitter Q&As where he fields questions?--?and dishes advice?--?on anything from running a startup to handling difficult relationship issues.

7. Answer questions on Quora

Charles Duhigg, staff writer for The New York Times and author of The Power of Habit, answers questions from readers on Quora on topics ranging from how to develop a blogging habit to what it's like to work as a journalist. 

8. Give bonuses in exchange for sharing a link to your book on social media 

Guy Kawasaki, venture capitalist and bestselling author, encouraged visitors to the site for his book about self-publishing, Author, Publisher, Entrepreneur?--How to Publish a Book, to share a link to the site on Twitter or Facebook in exchange for a bundle of free materials, including a book design template.

9. Launch a Twitter hashtag campaign

Novelist Monica Leonelle started a campaign on Twitter to encourage readers to share their experiences using the methodology for getting more writing done that she describes in her book, The 8 Minute Writing Habit.

Using the hashtag #8minutewritinghabit, readers tweet status updates about their works-in-progress. They also share details of how they apply her strategies for building a consistent writing habit by completing 8-minute writing bursts in-between their busy schedules. 

10. Give a writing prompt on Medium 

Web designer and author Paul Jarvis announced a writing prompt on Medium: "How do you give yourself the space necessary to create?" The question motivated hundreds of writers to share their thoughts, and helped draw traffic to his weekly newsletter and books.