Recently, I started scoping out the job landscape in Silicon valley. And holy cow!!! it's startups everywhere. Every tom, dick and harry is doing a startup! Which makes me skeptical of 99% of these startups, including some of those 'decacorns'.
What's going on in silicon valley? Are startups really making that much of money or just getting a free ride on investor's expense? Or is valley deluded with young-20-something-programmers who are just starting pet.com variations?
The startup boom in some sense has always been there. There is a bubble-like environment but I don't think it equates to 1999 -- most of the high-value companies are doing fairly well, having either good exits (Whatsapp, Tumblr, Etsy), or raking in decent revenue (AirBnB) or having marketing executives fight over the chance to advertise (Snapchat, Pinterest).
As other folks have pointed out, VC firms have gotten rich and there is money to go around so the amount firms are being offered is a little higher than normal especially for late-stage rounds when it looks fairly sure that the company is on solid ground.
The housing boom on the other hand is simply an effect of not enough housing. Startups aren't the major employer in Silicon Valley. The major employers are Google, Facebook, Apple, Microsoft, LinkedIn etc -- all employing thousands of people, all generating loads of profit and they aren't shutting down anytime soon; nor are they going to relocate anywhere in the near term. These people need places to live, but SV suburbs don't want to authorize extra housing, leading to the spike in prices. Even San Francisco they city has had way lower housing built than a city like Seattle over the last few years, though the demand has been far higher.
The housing boom / bubble will burst if there is a sudden, huge surge of new housing projects; but I doubt it.