Just from an advertising POV I think we still have billions and billions of dollars currently being spent each year elsewhere that will come online sooner or later. All that 'old media' money will surely push up more sustained activity online
The key word you used there was "sustained". Now unless you are talking about Google, Apple, Microsoft, Amazon, etc you're not talking about companies that have a track record that can justify the term "sustained" profitability.
Most online investments (even buying domain names) is "speculative" by nature.
So when big money starts chasing unproven models - you have a speculative bubble by definition.
That's probably true but if 95% fail and 5% each return 100 x your money, then it's worth playing the game isn't it?
And here lies the question - can you still get an ROI even if many of your investments fail. The majority of people never diversify enough to ensure this in which case the answer is "no" it's not worth playing the game.
The next obvious question, is a higher risk reward investment strategy more profitable that a more reserved strategy. Historically, when it comes to buying companies - tried and true companies make for better investments.
That said, it would appear that the average punter derives at lot of utility from the excitement of gamble or the chance to go from zero to hero, thus why we all love riding on a speculative bubble...
So it might be worth the investment if you are looking to add a bit of spice to your life.