I think it depends on what your goals are and waht type of site you have.
I mean there is no set limit of links you should have, more is better but 100 in one niche might be like having 10,000 in another.
I like the easy of blog commenting and it also means you can target similar sites to your own. Forums seem to be the way to get lots of links although a lot would probably be classes as useless.
I'm looking forward to seeing this thread develop and reading the new methods that appear.
Mike - Can you elborate on the whole facebook point you mentioned above.