They are just images with links, the same as on the community page near the bottom right. The new 2.3.3.2 version of mopjoportal now suports content templates in the blog so I created a template with this html fragment containing the images and links and this makes it easy to add them to the bottom of each blog post. I just googled for some free twitter and facebook icons and found those.
Note that while you can now use content templates in the blog, using javascript widgets like th jquery tabs or accordion in the blog might not be a good idea because they won't render in outbound rss feeds, but other content templates can still be very useful in the blog.
Best,
Joe