Twitter and Wikipedia were created more than 10 years ago. It was way more challenging at the time.
Just think about the infrastructure for example. There was no SoftLayer/AWS to save your ass. You had to handle servers/hardware on your own. That alone could use a (small) company.
It's not a complex engineering problem at all. Maybe it could be if it were done in the year 2000. It is not nowadays.
[Granted, there is still work to be done to make a decent product, they had great people and they executed very well.]