- you host at one of the Drupal-optimized services (Amazee, Pantheon, Acquia)
- you put a Varnish service like Fastly in front of the site
- you use BigPipe to serve the dynamic content
- you have experienced Drupal engineers available for your project
If you’re missing one of those pieces, I’d recommend pretty strongly that you find a different platform.
Bigpipe looks like it chunks data from the server to the client, but it's not clear to me how that helps with real time updates to the database?
Varnish can help to a degree, but if you're on a big provider you're going through their Varnish setup anyway (among other caches) and it's not super configurable.
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.
> 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.