Similar to my thoughts on licensing, the answer to this depends heavily on what you’re doing. From where I’m sitting, flickr would be entirely inappropriate, mostly because my audience isn’t there, but partially because flickr isn’t set up for science. Figshare is pretty awesome, partially because you do get a DOI for your work, which makes it citable) and DataDryad is becoming more popular. SlideShare seems to be the location of choice for presentations, though some people host their own. (As infuriating as SlideShare’s UI is…I can understand why self hosting might be attractive.) You’d want to make sure your location of choice supports the metadata that your field expects (or would like to see) to ensure your information actually can be viewed, read, and reused.
Twitter is great for less formal communication, but it’s not appropriate for a formal report. At the moment, unless you’re a rockstar, you can’t really say “I have a great twitter feed” on your CV when putting together your tenure application either. Facebook is similar.
One thing to keep in mind with nontraditional dissemination is to mind who owns your stuff once you publish it. Just like you turn over some (or all) of your rights to your work when you publish with a traditional journal, you might be relinquishing a lot of your rights when you post to various sites.