In the end, twitter didn't force Mubarak to step down, it was the generals.
The situation had become unsustainable and, to quote Cromwell:
“you have been sat too long here for any good you have been doing. Depart, I say, and let us have done with you. In the name of god, go!.”
The point about the twitter revolutions is that twitter, along with facebook, and the other tools allows people to communicate and to spread the message, and that these services use the same infrastructure, the internet, as everything else necessary to enable an online society. And as the experiment of turning off the internet in Egypt showed, a modern society cannot function for long without the internet. ATM's, online banking, trade, orders, and the rest cannot work. One might as well have tried to stop people writing protest banners by taking away all the pens.
In the old Soviet Union, the photocopiers were guarded to prevent their 'misuse', just as in early modern England (and quite a few other places) printers were licensed to stop unhelpful pamphlets being printed and circulated. Twitter facilitated the protest by allowing the angry and the disenfranchised to communicate and organise.
If they had not been angry, or if most people had been content, Mubarak would still be president.
Twitter allows people to give voice to their discontents. To quote Cromwell again:
Do not trust the cheering, for those persons would shout as much if you or I were going to be hanged.
Once something has been done once it can be done again.