Sorry, you can't disconnect from Facebook. An active Facebook account is required to use Spotify.
Only those users who registered before September 22nd 2011 can disconnect from Facebook. If this is the case, just use your spotify username and password to log in. If you registered after that date, you must keep Facebook active.
Nobody knows. The Facebook requirement has been around for 7 months and nothing has been done about it. Old accounts without a Facebook connection still work but we don't know what will happen in the future.
The requirement was made by Sean Parker, an investor in both Spotify and Facebook. He wants to tie music and movies into people's Facebook accounts so they won't leave Facebook. You are one of his "victims".
Spotify has a partnership with some companies that will allow you to create an account without a Facebook connection. I know two of these: TeliaSonera in Sweden and Finland, and Virgin Media in the UK.