[oblio]

I think your comment is close to the truth but not quite there. Companies don't open source their core business, basically an extension of "don't outsource your core business" (https://www.joelonsoftware.com/2001/10/14/in-defense-of-not-...).

Google: Search engine? Closed source. Ads service? Closed source.

Facebook: yes, the network is important, but still: Ads service? Closed source. Data science stuff they use for harvesting relevant data from users? Closed source. Dropbox: agreed.

[ishikawa]

I agree, companies like Facebook open source things that are being helpful for them, and in most cases, due to their huge scale, they didn't find anything similar and had to do themselves. For a company to open source its core product it only make sense when you expect to have revenue from enterprise SLAs and custom services, as it is the case of companies like MongoDB and Redhat.

[amelius]

I guess another reason for Facebook to not open source their social network code is security.

[rq1]

Nice try. :-)

[openfuture]

Security in this case meaning: "Keeping the truth of their business secure from the public in order to avoid backlash"