In the text it says "Post your progress on social media (use the hashtag #devtober!)" Is there a specific platform we need to upload to or can we just choose. 

The best platform would be Twitter for this :-) But I also saw someone last year who did his devlog at YouTube via a video and just linked from Twitter there.

Guess, this here also counts as social media? ;) Although, I do post at Twitter, too. And then there is my newly set up website. And Youtube, not to mention Instagram. TikTok and whatnot... I'm already overwhelmed and can't bear to post everywhere. So I stick with this community and Twitter. The rest is jus bonus... ;)