
I'm not sure how Google allowed you to advertise, unless you used smart keywords like editing or tutoring ; ) Facebook - same story. I doubt students these days trust ads. Most block them and don't see them at at all. In countries where ad blockers are not that popular yet, students don't have enough money to purchase such service.
There's also Twitter and Pintrest, but they are a waste of time too. From my stats, students use less keywords to find "writing services" than a few years ago.
One reason could be that there are fewer students now. Why? Because it's getting harder to get good student loans and having a college degree does not guarantee a great job.
Anyway, I'd try word of mouth marketing, maybe this good customer knows someone else she could refer your services to. Then, forums like this one which allows you to build brand (if you are a featured writer or something).
Paying for ads, knowing that 80% of them could be clicked by bots or your competitors, is not a good idea unless you have big marketing budget.