I was asked this a few months ago. There are various sources including Hackers News and Reddit.com. But reddit is just a collection of specialised sub communities (known as Subreddits) and even I wasn’t aware of quite how many there are.
I came across a link which lists 43 programming related subreddits. On the right hand bar there’ a list of a few Subreddits but at the bottom there’s a View More link to show all 43.
I’ve shown them here to give you an idea of what’s available. The member numbers will change and are often out of date anyway.