Loftyoz, that's kind of a tough question answer. A text-based social network like LinkedIn will use far, far less bandwidth per user than something like MySpace that downloads music, massive numbers of photos & graphics for every user page, etc. And, of course, your user volume will be a big factor.

I'd start off modestly and see how your bandwidth usage goes in the early days and weeks. Then, you can project likely bandwidth and server needs at higher numbers of users and adjust accordingly.

thanks Rogerd - do you think 300 gig at 1-5 Mbps is ok to start then? It will have Media downloads.

I don't do much with heavy media files, so I can't say whether that's enough. I'm a believer in starting small and adding capacity as needed, though. Unless you know for sure you'll have massive traffic (e.g., a tie-in with a really busy site), do a realistic traffic projection and start off modestly. Monitor traffic and bandwidth usage carefully and, if you see it taking off, expand capacity quickly BEFORE you need it. If you go for days or weeks with slow response, outages, or "exceeded bandwidth" messages, you'll kill the site before it gets going.

wow media downloads will put a load on bandwidth. One of my sites (NOT a social network) has gotten so popular that just the pictures alone beat that 300 gig. A social network with heavy media downloads will be HUGE bandwidth pig when it gets popular.

OTH forums and blogs use little bandwidth since it is mostly text, but they can use RAM and processes.

Media downloading sites will be challenging for a newbie, unless there is quick cashflow - and since its media, the CTR/CPM is likely to be low.