How frequently can you update your feeds?
Which medium do you feel more comfortable with?
What are your game's strengths?
You probably have a Facebook personal account, so it's probably the first platform that came to mind. Facebook is one of the best platforms out there for paid advertising (along with hip sibling Instagram.) The in-depth targeting options make paid campaigns really worth it - but if you don't pay up, you're very unlikely to even reach the people liking your page.
Advertising, native video posts, longer in-depth content, engaging your community. Don't forget those calls to action.
Very poor organic reach, long posts without catchy visuals don't do too well.
Twitter works like a conversation, and it's possibly the best place to link up with fellow game devs. You can share other people's work, ideas and show some personality.
Joining the game developers' community and game enthusiasts at large, showing work-in-progress (get those gifs on #screenshotsaturday!), extending your general network.
Visual and/or artsy games, event photos, WIP, showing personality in your studio life. The new Stories feature lets you engage in a timely way with your audience, making each of your followers feel like 'part of the club'.
Be prepared to spend a lot of time liking pictures on other accounts before you get a good following. If you don't have many art assets, you might struggle to post the recommended 5 times/week.
YouTube is a solid platform - of course, this is where you'll upload your trailer. But there are other uses for it that you might be missing out on. How about a development vlog, or showing off the art in your game, maybe doing narrative videos? If you can film and edit, you can really get creative in the content you offer.
Each video needs a careful set of tags, and video production to YouTube standards can get quite expensive if you want to invest in it.
Twitch is the biggest live video platform, which is just great as you can interact with the community in real time. As opposed to YouTube, there is no editing possible, so it really feels "true" - careful, there are no outtakes.
Snapchat is dynamic. It's quick, it's fun, it's spontaneous and most importantly, it's a *great* advertising platform if you have some budget. But this is very audience-specific, so before you get on board make sure your audience is on Snapchat.
Humorous games, or the very scary ones. If you have a young-ish (think 15-30) target audience and want a really personal relationship with your followers, go for it.
There's a lack of visibility that comes with it, because of Snapchat's temporary nature - and you need to be on the platform to see the content.
Tumblr is a great visual platform, halfway between a blog and a social network - I personally advise it for dev blogs when you're in a position to show some visuals for your game. Most of your content should relate to game updates in visual terms, but you can also make it more personal if you already have a newsletter etc.
Dev blogs, while staying as visual as possible. It will work best for games with a strong visual style.
Limited community-fostering opportunities, but the "ask me anything" feature balances it out.
If you're just starting out, don't pick more than 3 platforms, unless you have an in-house community manager. And if you're on Twitter and Instagram, feel free to connect with me there!