i just want to ask you about, which mp3 portals do you like most or which are the biggest or best in service ...
i just need some advice because i really don't have a clue about which portal has how many visitors or members.
This might not be quite what you're searching for, but I always found public MP3 portals to be pretty hard to embrace because of their lack of quality control. Most people I know find it pretty tiresome to skim through gigabytes of rehearsal recordings of amateur bands in order to eventually find some good stuff
I assume you'll not only want to search for new music but also release your own? In any way, you might be better off with a netlabel - there's a pretty extensive catalogue here.
My personal favourites are Magnatune (interesting concept, as it's not free-as-in-beer like most other netlabels - you can download everything as MP3 and purchase high-quality versions if you like it) and, for electronica stuff, kahvi (slightly biased on this one).