Perhaps you started a blog but have found that you don't have the time to commit to writing articles of substantive length. Or, you already use a variety of web-based tools and could benefit from an aggregated (and searchable) view of the data input or collected by those tools. An activity stream may be just the thing to provide the balance of time and value (to you and others).
An activity stream is more like a scrapbook than a journal. The variety of tools available today allow you to slice and dice your online "scrapbook" to create a web site or simply a feed to which others can subscribe.