For social media work, it really depends on the project. I’d recommend giving yourself an hourly rate and estimating how many hours it will take you. Do this even when quoting on a project-basis. Make sure you are happy with the amount of work and how long it takes you to do it. I’d recommend starting with $15 or $20 an hour depending on the average hourly rate for the service and your level of proficiency. You can then increase this as your experience grows.
In my first few years, on several occasions, I underestimated and I probably ended up working for less than the minimum wage most of the time. Now, I overestimate but I also overdeliver on my work output. I find I always work to the best of my ability if I feel valued.