> if you’re planning to be a top 10 site on the internet, you’ll need a custom stack.
And yeah, that's what I thought the OP was hinting at (or at least hoping to become).
It'll handle small-medium scale forums just fine. Probably any stack would.
To be fair, that's the case with pretty much any CMS. Even if they start with a stock system, by the time they actually hit top 100/1000 (let alone 10) there's a lot of custom work to be done.
And realistically, a "top 100/1000 site" is almost always going to have a cloud of other sub-sites and related properties. The corporate site, intranet/HR, jobs and recruiting, press center... The penumbra of "Oh, yeah, we need one of those... but it would be ridiculous to build it from scratch" sites for a large organization can be pretty large indeed.
I’m not sure I read the OP the same way you did. If you read it as “can I build a site on the scale of Reddit or Twitter with Drupal” then no, don’t build Reddit or Twitter with Drupal. If you read it as “will Drupal serve a site with user generated content” then yes, it will, but if you’re planning to be a top 10 site on the internet, you’ll need a custom stack.