If you plan to expand to that level, your talking several file servers alone, not including multiple load balanced web servers and at least 1 dedicated sql server. Just the top 20 alone is almost 6 terabytes of data. Just to give you a rough idea, i priced it at 2 places. the first prices a basic file server with 4tb storage at 389/mth with a core 2 duo and 2gb ram. youd need at least 2 for just the top 20 which would leave 2tb left. add in a mirrored raid setup and you would need 3 for the top 20. assume a mirrored raid and a minimum of 20tb storage, youd need 10 servers for that at 389/mth. id recommend doing a colo on a 10-20mbit line for that with your own home built servers. the second place will only allow 2 1tb drives and thats 359/mth for the same rough setup. bandwidth is another issue as most offer 100mbit lines for burst abilities but only 2tb of bw/mth.
assume the first host, 10 servers, 389/mth, 3899/mth overall, just file servers. adding an extra 33tb/bw/mth is 1800, so thats 5700. that might just barely clear your needs for file serving if you plan to be that big. place in 2 or 4 web server, 1 sql server, load balancing, assume 2 web servers at the next level of processing power with some extra ram (q6600, 4g ram, 250gb hddx2 for raid), 259/mth. do the same for a sql, another 259/mth, ignoring cost of load balancing for now, your at almost 6500/mth just to run a basic version.
figure if you do a full rack of servers, its 1350/mth, plus bandwidth, 1800, so 3100. build your own servers at 2k/ea (just an example), 10 file servers, 2 web, load balancer, sql, 14 servers, 31100 upfront cost. then 3100/mth after. (plus a switch, a nice cisco maybe) but all in all, thats 3400/mth saved, in 11 months, thats 37400 saved, so you end up saving yourself a little money in a year.
for colocation, another place lists 1475 for a full rack, they only allow you to upgrade to a 10mbit line, totals 1975. but you only get 3tb for a whole month. youd need more. so they’d probably quote out more then the other place if they did 33tb, about a 100mbit.
But also, look around, those are 2 larger providers. A local one to me is Calpop, they have a full cabinet at 899, plus 1000 for a 100mbit unmetered. so 1899, plus 10 ips is 1998. so less then the first, and less then the second if they offered 100mbit.
go colo, not shared or dedicated. in terms of cost effectiveness, you need the most bang for your buck. you fall into a disadvantage with colo. chances are high that its unmanaged, you and you alone are responsible for it.
consider this as well, one bad file that is reported to the host, they locate it, they disable your account and chances are (shared, dedi or colo), no refund. so doing a file sharing site is usually a bad idea if your not going to have at least 20 full time site admins checking files as they come in.