Was Viaweb First?
[1] http://en.wikipedia.org/wiki/Amazon.com
[2] http://en.wikipedia.org/wiki/EBay#History
Viaweb was not an e-commerce site like Amazon or Ebay. Its customers were merchants rather than consumers.
If you define it more narrowly as "something that lets you create a site", Geocities still counts.
I think you'd have to narrow the category to "first online store builder" to make it accurate, which is still quite an accomplishment, but certainly isn't the "first web app".