Its my personal opinion that pictorial representation of a concept is icon , and is widely recognised by, and you don't need a text supporting it more of pictorial.
where as
logo are concepts and they are abstract, which may or may not require text to support and needed to be made recognized by users for certain concept, there are some might make your brain to think what it made for else its easily understand . at times a pictorial representation of charters also. like a person name. if we talk about organization they too have name and we symbolizes that those few characters. like Bloggers "B". The name in it self is so big that you don't need to give an organization feel to it. If that is very popular, but if its not it becomes a tough one to achieve that, but for all of us now we are happy with the same pattern. ie convert characters to icon. make big T or Big R.
there are some intelligent logo making too. very sharp with there thinking. they blend it and put a little curiosity to recognize it but when you do it you really appreciate his thinking. i will send out some links to such creative logo making. I think they are complete.
if you look at other logos then there is a issue like you wont be able to make it out what it made for unless its supported by text this is a fact its really hard to achieve such a thing and it will be widely accepted. like blogging websites you take out icons of such blogging site, do you see some pattern.
i guess not. though they belong to one generic concept blogging. but we move one step deep and tried to provide identity to these abstract concept which will be blogging site specific and blogger on goggle and blogger on yahoo... its all different.