My view is a little more nuanced, and is based on anecdotal evidence and conversations I've had with VCs and investors in venture capital funds. I'm of the view that in general, we are not in a bubble (as compared to the Dot-Com Bubble), but we are in an elevated state of valuations in certain sectors. We've recently seen some impressive acquisitions (Whatsapp, Oculus VR, etc.) at sky-high valuations, which suggest these sectors may have bubble-like characteristics. I hear from VCs that initial investment valuations are very high in certain sectors, which makes investing difficult, but that given the high valuations now is a good time to be exiting investments. The encouraging thing about this for me, who was active during the Dot-Com Bubble, is that there is a recognition now that valuations are high, versus the excuses I heard in the Dot-Com bubble about paradigm shifts etc. that were used to justify ever-increasing entry valuations.
So I don't think we are in a bubble overall today, but that we are in a period of elevated valuations and certain sectors have bubble-like characteristics. This situation does bear close monitoring, as if venture capital fund raising levels spike up, then more money will lead to higher entry valuations and exits, which could easily lead us into a bubble.
Business Insider article: http://www.businessinsider.com/historical-venture-capital-funding-2014-4