Also, you can't create a product in this short amount of time. (There are some websites that do almost nothing at all, and that are very profitable, but those aren't engineering problems). Think how long git has been maintained, polished, and extended since its inception. And still people complain.
Software is a learning process, otherwise people wouldn't be so obsessed about it. And learning takes time. No matter which way you're approaching it.
Other part about Facebook - well if you take first version that was just well photos + some description and disregard all stuff that was refined next years where it started to be real product instead of novelty page for Harvard students.
In a company I work for we built first version of application in 1-2 years to catch up with what other companies already had and what customers expected. We could do really simplistic version in a weekend but no one would pay for it.
Genius!!!
And git wasn't even built in 10 days, as Linus has said elsewhere. The core structure was, kind of. But there's been a large effort on top of that.
And the idea that Facebook was built in a weekend is just completely wrong. Sure, some basic Facebook-ish app was created over a weekend, sorta.
It doesn't take long to "build software" - git was built in a weekend, Facebook a similar timeline. When the environment is right there's no upper bound on the pace of delivery, it's just that modern day dev is, in many places, less about quality of the output (in terms of product/market fit and user value, not code quality) and more about the satisfaction of ceremony. You couldn't product manage Google into existence, or any truly valuable piece of software, hardware, or any other innovation, but it's customary for organisations to create structures which feel grown-up in order to yield software.
The talents now required to enter the field are also different, which has naturally played a part in the perceived slow-down. Development is a new field, and research into what makes teams effective is limited - we're still using the ideas of industrial steel production to try and bolster the present manufacturing line. This means there are plenty of undesirable behaviours.
But ultimately, it's about what you're optimising for. If you hire outstanding people and give them an excellent brief, you can deliver incredible software at pace. Most modern companies don't optimise for this, it's more about satisfying sprawling teams, polishing egos, and chasing incremental gains. In that environment you can bet delivery is slow.