You're not doing what you were hired to do
We all have new job responsibilities added to our plates all the time. It's the nature of the digital marketing biz, and that's OK. It's how we learn and grow. Eventually you might learn enough and grow enough that you find yourself in a completely different role at the company. That's fine. That's what we call advancement. Congrats! What I'm talking about is when a company hires you under the expectation that you will do one thing, but then hands you an entirely different job as soon as you get settled at your desk.
If you thought you were being brought on board to develop and build a brand's social strategy, and instead you're sitting at your desk proofreading and automating tweets that you didn't even have a hand in writing, something went wrong. Your company did not set the proper expectations or -- worse -- it changed things on you without informing you. You thought you were going to be able to stretch your creativity, show off your stuff, and build your portfolio, but instead, you're trapped in the mundane, with no indication that the situation is going to improve.
If your company pulled a bait-and-switch on you, that's not cool. Your career should be on an upward trajectory, and if you discover that you just unwittingly took a step back in a role (even if the paycheck is the same or bigger than your last gig), you shouldn't stick around. You could ultimately be setting yourself back for years to come.
Of course, this works the other way, too. Did you enter a role with particular duties in mind, only to suddenly find yourself running an entire department with limited support and working insane hours? If you're enjoying the job and your paycheck acknowledges the crazy workload, that might be OK with you. But if your company hired you as a manager, at a manager's salary, and then slapped you with director-level stress, it might not be worth waiting around for a promotion that might never come. Don't discount the stress factor. Burnout is a bitch.