value and valuation
It's really wonderful what you can do with the hello-world of network programming, a text chat server. The core Slack functionality has only been implemented about a million times in the last 30 years. I guess that means it's ripe for a Yahoo! purchase!
Seriously, it's a nice, slick chat server. One of the best ever implemented. And it understands Youtube and Vine links! And and-- well anyway, there's got to be at least 100,000 lines of code in there. Probably a good four or five engineers responsible, not to mention a designer. Definitely worth $5 billion.