Small blurb posted to Facebook

When a user logs in, the text that's posted to their wall comes from the Status / Wall tab of the configuration area. In there, you can set the message.

IF you're also using a URL in the Status / Wall area, then you'll want to make sure you've enabled Open Graph integration (Social -> Open Graph tab). Then, on any pages with Like buttons, comment boxes, or other ways that'd it'd be posted to Facebook (like the login message), you'll want to use the Open Graph system to describe that page to Facebook. You can do so by adding the following tags to the page:
{JFBCGraph title=Title for Facebook}
{JFBCGraph description=Description of page when shown on Facebook}
{JFBCGraph image=http://site.com/blah.png}

Additionally, in the Open Graph configuration area, you can set defaults that will be used across your whole site in case you don't explicitly get those tags on the page.

To test how Facebook 'sees' your page, you can always use their Linter tool which will scan your page for those tags:
developers.facebook.com/tools/lint

Hope that all makes sense, and helps get you started, but if you need more help, let us know.
Alex