Hi, I'm a Sprint employee and Epic 4G owner (for about a week). I have had no issues syncing FB contacts with my phone contacts. I know you've installed the FB for Android app but you may have to make sure you've chosen to sync FB contacts, it may not be chosen by default.
Menu > Settings > Accounts & Sync > Facebook account
Make sure "Sync Contacts" is checked. If in doubt, uncheck and re-check the option. The sync should start immediately.
You may have to go into your contacts and manually link some of the contacts with their appropriate FB acct. My Epic has done a far better job of marrying phone book contacts with FB contacts than my last HTC Android phone, so I'm impressed by how seamless it is.
Hope this helps. Thanks for being a Sprint customer and Epic owner!