This is a tough question to answer because there isn't a single best platform to market on. Instagram is owned by Facebook, so you technically can run ads on it through Facebook Business Manager too! Also, with a product like socks (which everyone wears), I'm confident you can find an audience on either.
In terms of which one I would try first, I would suggest really breaking down your budget. If you have some capital to invest, Facebook ads are great for mining for data and really pinpointing your audience. If you don't have as much of a budget to start with, Instagram can be great because you can build up an organic following and interact with your potential customers. It will take longer, but it will be more self sufficient in the long term.
In our Dropshipping 101 eBook we have an entire chapter dedicated to various marketing channels. I would suggest giving it a read as it might give you some insight into what you want to try first.
I say try as well because the first option might not work for you, and that's okay! Many dropshipper find success after trying a second or third channel to advertise (Google Adwords or Pinterest for example). Hope that helps, all the best!