The journals used by the students at Hogwarts began as an invention to keep in touch with loved ones during the stressful times of war. However, it soon became apparent that the crude magic of the journals was only useful within a certain range -- and the students were left to talk only to each other.
.magic
Identification: The journals are imbued with magic that will identify the writer of an entry or comment, no matter what sort of disguise he has on (similar to the identification charms that will one day be used on the marauder's map by four very bright Hogwartseans). There is no way to circumvent this except to ward a comment or post private.
Wards: Any post or comment can be warded private, or warded to specific people - IF, and only IF the warder knows exactly to whom he is warding. One could ward, for example, to "the Slug Club" if one is IN the Slug Club, or knows who the Slug Club members are. A student cannot ward to "purists" because he has no way of knowing everyone who identifies as a purist. You must have a wand to place a ward but not to read it. However, a person can ward a wandless readers comments if he or she refreshes the ward for every new comment.
Because all journals can recognize the reader or writer regardless of which journal one is reading in, wards work for the person not the journal. One cannot look over another person's shoulder and read through the other person's wards.
Warded magic cannot be broken via another journal, as the wards are performed on words in the original journal. Unless a person knows a ward is present and has possession of the original journal, there is no way to alter or affect that magic.
Multiple wards: Due to the uncomplicated magic on the journals, wards can only be performed once per entry and once per comment.
.mechanics
Layout: Much like the TARDIS, wizarding journals are larger on the inside than on the outside. They appear from the outside to be narrow books - maybe 20 pages in size; once open, they can be many hundreds of pages - however many are needed to contain the journal entries of the person writing in them. In order to preserve sanity, a journal will show the last 20 entries a person has written, and if the person desires to read further back, they must be conjured up - replacing the current pages. A set of pages in the back of the journal are designated 'others' pages, and will show only 14 days worth of entries from others with a journal.
The 'Ink': Because of the nature of the journal magic, ink is not static and therefore cannot be erased (only scratched or torn out), or captured by muggle devices. A muggle photograph will show only the pages of the book, devoid of all information. A magical photograph will show only the text to which the person viewing the photograph has access (i.e. that which is public or is warded to them).
Range: The magic is just strong enough to communicate with other journals a few miles away. This means that Hogwarts/Hogsmeade are isolated and cannot communicate with journals outside of the area. When students go home for the holidays, they can only communicate within a few miles (surrounding cities-ish).