This is one of those nicely ambiguous counterfactuals. Contrast, as David Lewis does:
If Caesar had been in command [in Korea] he would have used catapults
If Caesar had been in command he would have used the atom bomb
In this case I am thinking about catapults. I mean Twitter is obviously a much simpler system than the web and in that respect it might have been easier for a researcher in 1992 to dream up a good way of sharing citations amongst internet-connected scientists. Also the grammar of Twitter is much thinner, less ambitious and more constrained than the grammar of HTML. Precisely because Twitter is a lot simpler than the web, it would seem at least possible that it could have evolved sooner than it did, just as catapults came before atom bombs (OK -- I know that we didnt have SMS in 1991 so it wasn't there to be canibalised, but please don't spoil the fun). Let us suppose that Twitter had come first, that Tim Berners Lee had come up with some consistent, open, distributed, asymmetrical protocols for sharing references and very short messages across the internet. Surely that was the kind of scientific communication tool and update system that the CERN bosses would have been looking for? They could have given him a straightforward promotion. Why did he have to give them so much more? What would be so different with the way the internet is now if he had given them less?
The web would be a lot smaller. There would not be so many big files. So many big web pages. The web would have been very different if TBL had limited a web page to 140K. The web as a Twitter-verse would presumably be a lot flatter, more horizontal. Fewer pools of database-driven depth. There would still have been spam (Twitter has about as much spam as the web). There would have still have been porn and nice graphics (Marc Andreessen could have saved himself several man months and invented Twitpic, which would have been much easier than writing Mosaic). There would also have been much more indirection (bit.ly, tinyurl would have taken the place of Yahoo and Google) a lot more opacity (domains, countries, languages being harder to organise in the Twitter framework). TBL would have found it as hard as the Twitter founders to discover a business model for the invention (that is the penalty for inventing a new syntax). But it could still have spread like wildfire, as indeed Twitter has spread like wildfire these last three years....
However it is a good thing that Tim invented what he did, catapulted us into the 21st Century with his super-collider of an invention and left it for Dorsey, Stone and Williams to pick up the other simpler, similar, idea. Later. These counterfactual musings are prompted by yesterday's announcement that Twitter is now developing a new @anywhere service layer, which suggests to me that Twitter will become an even finer-grained and more diaphonous network, parasitic on the web, but aiming to interconnect as many web resources and web services as possible in a layer of commentary and shared perception. It is as though the Twitter founders are trying to fill the conversational interstices in the gaps left by the operation of web services. This proposition may have important implications for publishers and media.