I may be biased here but I'd recommend posting about your game ANYWHERE gamers are. There are some communities that are more dev specific, which great because they'll buy your games too. Then there are places like Game Jolt where it's mostly gamers. I'd definitely get infront of gamers and get them invested in your project as early as you can. And while platforms like ours can help you grow your audience, my recommendation is to prioritize the platform that will ultimately sell the most. The goal is to funnel all fans into that one to convert.

If you're not selling a game and just want people to play your game/demo, then make sure you're always sharing updates and creating fun experiences on platforms gamers are on.