7 years isn't necessarily too long to bring an idea to fruition. It's out towards the edge of the distribution for an internet startup, but it's perfectly reasonable for something complex.
Maybe in the corporate world, but if you are small startup then surely not.
The world changes so rapidly and most of what small startups offer are solutions to a current need. If you are doing it as a hobby or for fun/learning/charity then that is different, but as a business (the purpose of a business is to make money, and startups are businesses ultimately) it is unfeasible. As a basic proof, try pitching to investors/VCs. Even friends/family would be reluctant to invest as a business.
Not every startup is a web application built around a CRUD model, seven years is a short time for bringing a new medical device to market; and a long time for a novelty ice cream flavor.
Time to market is not always as crucial as it seems; photo.net was doing photo sharing and social networking ten years before flickr got bought by Yahoo!, it's more than possible to be too early to market.
You don't go to VC's during the R&D phase, you go to them once you've worked out the technology risk and have something that could be taken to market with a sufficient application of money.
It's a good thing that there are inventors with long attention spans because we would still be chasing wildlife for dinner if there weren't.
Also in that amount of time, technology will have changed significantly brining new issues (e.g. the need mentioned to make it look "web 2.0") and opportunities (for them and for others).
Even without knowing the details and complexity of solution, I dare say unless the value is in core algorithm/architecture design that can be migrated, much of what they have done will be outdated (based on assumption the technology-stack/tools/learning/knowledge they use is now getting outdated).
Twitter is not simple service and faced many unique problems much discussed already here and on the web in general.
Also you should not be basing your offering on what the current hype is, but rather on filling a need and offering something people want and will use. When the hype fades, atleast then you will (hopefully) have something of value that will last beyond it.
The world changes so rapidly and most of what small startups offer are solutions to a current need. If you are doing it as a hobby or for fun/learning/charity then that is different, but as a business (the purpose of a business is to make money, and startups are businesses ultimately) it is unfeasible. As a basic proof, try pitching to investors/VCs. Even friends/family would be reluctant to invest as a business.