On the Rover Dashboard, there are direct links that are helpful when posting (advertising services) to other social media sites, such as Facebook, Google+, Twitter, and NextDoor. To help encourage you to do so, Rover even regularly offers sitters giveaways to earn points redeemable towards gift cards (Currently: Pick-Your-Prize Spring Giveaway).
I also use business cards to spread the word. I've given them out during walks/outdoor play time, shopping, etc. I also found that some independent/local smaller chain pet stores (not the biggest national chains), groomers, and veterinarians have places where they'll allow pet sitters to leave cards. You may also distribute cards at dog based events, and find other businesses/organizations too.