Many 2 Many mentions some new statistics on the use of the terms blog and weblog. As they note:

“Blog” outpaces “weblog” in 2003, 687 to 389; that’s a big change—in scale as well as preferred term—from 2002, where the respective numbers were 270 and 274.

Source: Out of the Crooked Timber, “Blog coverage” via Many 2 Many

I’ve found most often that if I use the shortcut term blog, I generally have to backpedal a little bit going to weblog as the next step. Assuming that point of explanation fails, then “online diary” seems to get it clear. What are you using?