[cft]

Those ones (along with Google and Microsoft that are arguable hard tech) were founded prior to web 2.0. Since about 2002 no hard tech software startups achieved any scale- they are sort of all "moving up the stack". When they emerged (Android Inc) they are promptly swallowed. In the mid-2000s there were Yelp (from the software perspective, a simple website with Lucene backend and Youtube - a simple website with Flash player and ffmpeg transcoding command). It's probably a sign of consolidation of the industry.