I think it depends on the situation. For example, if you see them all the time, I think it would be nice to ask them in person, or, if you want to do it over the phone, I think it should be a specific call to ask. You shouldn't just drop it in the middle of a regular conversation like "Hey, how are things? Do you want to be a bridesmaid? Have you seen that new movie with ___". But I think "I wish I could have asked you this in person, but you are so important to me, I didn't want to wait until we saw each other in person again to ask. Would you please be my bridesmaid?" is ok.
I would avoid online if you can, but that's just me because I would want to be able to talk to my friend about it, share in the excitement of it all (whether as a bride or a "maid"), and that's harder to do online.
at 5:06 AM on Oct. 22, 2008