For whatever reasons, it seems to me that relatively few black people study for engineering and science degrees. At least that's my feeling in Europe. Then it's simply a trickle down effect, like for women in tech: Few choose this path so even fewer end up being tech entrepreneurs or at Google/Facebook, etc (since the article mentions the workforce of these companies). So, once again, education from primary/secondary…
There are many, many non-technical founders. Most jobs at Google and Facebook are not in engineering. A lack of blacks in computer science might explain why the engineering department has only two black employees but how do we explain the sales team?
Old school. If you needed another team member HR and the Boss would hire one. And you'd have to work with them. If the company thought they weren't hiring enough women and blacks they'd just hire some women and blacks.
New way, we only hire from the top schools and programs. And we allow members of the team to act as gate keepers over vague things like 'cultural fit'. Over time your team becomes composed entirely of upper class whites and a few model minorities.