We regularly post ads for designers, developers and content writers too. Probably every other month I am reviewing resumes, so I have a bit of experience in responding to applicants.
Typically any given job posting gets around 50 to 100 resumes depending on the category. It takes time to read through everything. We use a mix of Craiglist, Indeed, various job boards, LinkedIn and our own website postings. It has worked well for awhile now.
Due to the nature of various job boards, some responses are often automated by clicking a few buttons so it is too easy to apply. With that you get candidates off topic (i.e. wrong skill set) and not putting in any effort (i.e. skipping a cover letter that we want to read). Other times, the cover letter and resume are comically sad in mistakes and grammar, so they do not warrant full attention.
Our biggest gripe is that at least 50% of the applicants never follow our instructions on how to apply correctly. We put very specific steps on how to apply and want it followed (i.e. subject line, cover letter, links to portfolio, PDF resume).
My take on this is if the applicant is not going to take the time to respond correctly, then we are not going to take the time to respond back. Sounds harsh, but kicking out 50 canned replies is tedious work for those who are wasting our time to review. Perhaps we could automate it somehow with scanning emails addresses in the "no" folder and sending out a batch bcc, but it is still extra work.
However, before everyone jumps on us. We do respond to every applicant we interview (phone or in-person). Often we respond with a personal note that helps them out and lets them know where they stand. Sometimes we even try them out on freelance projects in the future.
For every applicant that correctly fills out our application process, or at least made an effort, then we also reach out to them with a more automated message via email. I consider that the least we can do, but we do keep in short and sweet.
Finally, in the future, I am going to probably put our entire application process on our website. This will automate a lot of what we do and require the applicant to fill out set fields that we need answered. Then we could send batch emails back to everyone as needed.