Principles of persuasion
Cialdini's principles still apply to online communications. People will need to implement them differently than they do in person. The most interesting limitations have to do with transmitting information, instead of physical things.
Generating an obligation, for instance, is more difficult with an email message; mints do not download well. I have thanked people by 'buying them a beer' through paypal, in the past. Some variation on that theme could work, but would be expensive. This would not be an appropriate mass mailing tactic.
We can evoke a pattern of consistency just as well electronically as in person. Public responses such as tweeting or liking will be more effective than private answers.
Senses of scarcity and liking will also be very similar. It will be harder for us to conjure authority. A simple statement of credential might work, but if placed early or poorly, might prime the reader to resist the persuasion.
Authority and consensus are tied together online, in many cases. A message tweeted from a fashionable source partakes of both factors.
The most important limitation of the online channels of persuasion is their anonymity. An agreement can be unmade as easily as made.