If you go for the shared strong secret part "uniquefied" by an added per-site trivial part (not saying that you should), you increase risk the longer the trivial part gets: if an attacker somehow determines that your password here is 123hacker456news789 they will easily guess that they can get into your Facebook using 123face456book789. Less easy if it's 123h456n789 (Because you don't really use the spaceballs password, in reality h and n don't stand out half as much). If it's 123c456w789 guessing Facebook's 123c456o789 from that would be quite close to brute forcing unless they get their hands on dozens off that kind.