If companies get swallowed up by the Google/Facebook behemoths like so many do then investors usually get their exit, to make an example of 2 from your list Tumblr was bought for $1bn by Yahoo and Snapchat allegedly had multibillion dollar acquisition offers from Facebook and Google. Apparently the megacompanies don't buy these smaller companies for the value that could be generated by revenue streams from them(when and if), but more a combo of the "strategic advantage" - so their competitors don't snap them up first, or out of a fear that one of them could end up being a direct competitor to them one day, as is the case with the Facebook Instagram acquisition and attempted Snapchat acquisition.