Well, there are a LOT of things that can reveal one's username and password. Here are a few (some obvious):
1. Allowing a third party access to said account (obvious)
2. Using a Proxy/Anonymizer-type site for access. This may SEEM safe, but really isn't. Your details (all of them) are logged on the proxy's server. SOME of these sites are operated by people who wish to retrieve your login details. This is actually LEGAL, since the user is volunteering his/her information to what is, essentially, a complete stranger. Keyloggers are unnecessary in these cases...
3. Keeping a written list of usernames/passwords in a public/readily accessible place. Also obvious, especially when at work OR at home with tech savvy minor children.
4. Allowing a BROWSER to save your login details. Browsers are routinely hacked, regardless of which brand(s) you use. This is particularly true if routinely downloading warez (even from trusted sources).
5. NOT running MULTIPLE virus scans after downloading/installing anything from a person you do not know personally... (obvious)
6. Making dang sure your virus scanners are FREQUENTLY updated (say, twice per day). Hackers are very adept at creating new sequences once their older ones are detected.
7. Periodically backup EVERYTHING important on your computer and then wipe your hard drive(s) at least twice a year (four times per year would be optimal). A reformat and clean install of your OS will prevent any undetected viruses/keyloggers/etc from using your system. Might be a PITA, but keeping your details safe should be worth it to you. This also goes for external drives...
These are just a few, as stated above. If you MUST keep a written list of your various account accesses, then I suggest putting it to Notepad and using TrueCrypt (for example) to protect the file.
Premium account access details to ANY filehost are routinely SOLD. It's really not that difficult to get hold of account details via third party sites (such as #2 above).
On slightly different matters:
It would be wise for FS to seriously consider a bit more security for questionable accounts. It would also be wise to link any suspensions to auto-cancel recurring payments AS WELL AS allowing self-cancellation for current account users (assuming abusers of such a system would be prevented from signing up again).
Please, folks, understand that FS is experiencing, right now, what are generally called "growing pains." They may have expected rapid growth, but I'm assuming that it turned out to be more than they dreamed. RickyFS might be caught in that unenviable business cycle where he is overwhelmed with necessary duties but not yet rakes in enough regular business income to sustain full time employees to handle customer service. From experience, I know how difficult customers can be.
Be patient, and understand the above points (and more). The Internet is still the Wild West. There are a myriad of ways to compromise someone who thinks they're safe.