This has happened to me a few times and doing the following one or more times seems to fix it without having to delete and re-add accounts:
1. Menu -> Settings -> Accounts & Sync -> Add account -> Facebook for HTC Sense -> Sign Out (not sure if it's the same for Twitter but do it there too)
2. Either return to that same screen and sign back in, or go to FriendStream and follow the prompt to add an account
If it doesn't work the first time, wait a while and try it again. Make sure to go back into Accounts & Sync -> Facebook for HTC Sense and check the "Sync Live feed" and "Sync Contacts" boxes!