Tumblr didn’t have a dedicated server administrator until a few years into its life, so I had to be our de facto server administrator in addition to my primary developer role from Tumblr’s start until we had 48 servers handling about 5,000 requests per second.
Since that’s a sizable infrastructure, but we had very little time to devote to maintaining it, I had to choose mature, stable technologies with great tools and very low administration needs. And I built Instapaper on the same technology: PHP 5 with a custom high-performance MVC framework, MySQL, Memcache, Apache, and CentOS, on high-end dedicated hardware.
This proved invaluable to both Tumblr and Instapaper, since I don’t need to deal with webserver processes crashing, corrupted databases, or any serious platform issues. All of these tools are used by many other companies with deployments much larger than these, so nearly all of the bugs get worked out long before they get a chance to affect us.
The result of all of this platform conservatism is that I can spend my time improving the product in meaningful ways instead of fighting with my server software.