During the lead up the 2008 debt crisis I saw a lot of people talking about how house prices had been going up year on year and questioning whether we were in a property bubble. There was a debate though. Demand for housing was strong (partly due to population growth) and it appeared that globalisation had freed up the capital so banks had plenty of money to lend to people to buy houses. So the rising price of property, it was argued, looked like the natural effect of market forces. In hindsight it's easy to see that there was an oversupply of wholesale debt, but at the time I think people assumed that it was just a more efficient distribution of global capital that was opening things up.
Similarly, in the lead up to 1999 people seemed to know that there was too much money floating around. Everyone seemed to be investing in stocks or property and swapping stories of how much they had made. Yes, tech stocks were rising quickly, but so was everything else, and there seemed to be no shortage of investors coming up with money to fund these tech companies so what's the problem.
I think what happens is that people always look at current circumstances in the context of recent bubbles and try to look for patterns. When they don't quite fit, they argue over whether this really is another instance of a previous bubble. My theory is that the overpriced asset is rarely the same thing as in the last 3 or 4 bubbles.
So, back to the present day. We have lots of high valuations for tech companies. But we also have lots of tech companies actually making a profit, plus operating costs are a lot lower. So we're not about to see a burst in the sense that the funding will dry up and companies will run out of runway. Instagram was like, 12 people(?), if they couldn't have got that $50mil investment I'm sure they could have found a way to keep operating for another year or two.
The challenge is to think about where the bubble might actually be. Could it be in advertising revenue? Are advertisers burning though reserves trying to get attention? Could it be in mineral resources? Are server or bandwidth costs artificially low? Could it be a bubble in intellectual property? Is the exact implementation of Facebook not actually as valuable as the companies valuation suggests. I have no idea, but I don't think this a complete re-run of 1999.
TLDR; I don't think this is "[Tech] Bubble 2.0", this is probably "[Something else] Bubble 1.0".