ArtStation is the most popular platform. Most artists use it as their online gallery of works. 

The major advantage of ArtStation is the number and quality of its users. You can ask a large global studio’s lead artist for their opinion, or receive feedback from a pro in another part of the globe. The platform is also very helpful for networking. Most artists here are always ready to give advice and help.

ArtStation has a built-in blogging feature that is popular with users. On the blog, you can publish notes, articles, and WIPs, thus expanding your outreach.

The greatest disadvantage of ArtStation is the challenge of receiving comments if you’re just starting out. In order to build your audience to receive more real-time feedback, you need to develop and market your account first.   

PolyCount is a large CG and game development platform. It has a main news block, a forum, and its own Wiki. The website offers artists’ ratings, definitions of key industry terms, and article reviews for a variety of projects. But the most interesting thing about it is the artists’ forum. 

PolyCount is divided into several sections and subsections. Some of them allow requests for feedback. It works very simply: you register on the platform, create a project page, publish intermediate and final results, and then wait for users’ evaluations.

Unlike ArtStation, you don’t need to have a well-marketed PolyCount account to be noticed. The platform is a community of experts, so the chance to meet incompetent, impolite or aggressive commentators is minimal. As a rule, users try to give valuable, constructive feedback.

Facebook, Telegram, Discord, Instagram - all of them can be useful. Here you not only have communities with memes and CG news, but also specialized groups where you can publish your work and collect feedback. 

The major problem with such resources is user diversity. If the previously mentioned platforms host mainly professional art, on social networks your risk to meet incompetent people is higher. Yet even in this big ocean, there are islands where you can get competent feedback and find new

Other social networks have the same search principles. Groups are distinguished either by software or by work specialization: 2D/3D, character art, etc. There you’ll easily find what you need.

The best way to get competent feedback is mentoring. Find a person who’ll agree to review your works, answer questions, and indicate where to find relevant information, and your professional development will accelerate in leaps and bounds. 

This option is more effective than others. First, you can receive feedback at any stage and for any issue. Second, your mentor will keep track of your performance, notice your weaknesses, and instruct you on how to cope with them. 
The easiest way is to find a mentor is among your acquaintances, in specialized chats, or on special platforms. You can offer compensation or, if they agree, even negotiate free feedback. But keep in mind that your mentor is human and cannot respond to your every whim. So, use this opportunity wisely.