When addressing a group that is unaware of its membership (a blog, a BCC, a mailing list), it is often best to use a personal voice and address the group as if it were an individual. Avoid phrases like “all of you”, “sorry for the mass mailing”, or “some of you may know”. These phrases tend to distance the recipient from the message, and make it harder to convey emotional content.
On the other hand, do not try to intentionally trick recipients into thinking that the message was only sent to them and no one else. In some cases (blogs, message boards) the platform itself will make that obvious. In other cases (BCCs, mailing lists) use subject lines and headers to hint at the one-to-many nature of the message.
Reply one to one to follow up conversation to the extent that is possible, even if the reply is brief. Extend the personal voice to these conversations as well. Avoid referring to other people's responses with phrases like “You are the tenth person to say that”.
This sort of language breaks the promise of the personal voice.