I would tell them online, then if they're unsure of what to say they can take some time to work out how to respond/get their heads around it
perhaps try and think of what you want them to do, if anything, so that they don't worry about it or feel that they need to do something to accommodate you when it's not necessary - try not to be too demanding, remember they don't have an investment in you yet, but I think the idea someone else had about suggesting a pizza/movie night (perhaps on a boring looking night?) early is a good idea as it will also take the pressure off them if they feel like there's a plan to involve you in... also think if there are things you can do together in the day e.g. go shopping and split a taxi or go into town and find somewhere to get breakfast or something?