It's easier to look for an installation of WordPress (or whatever) with an exploit than it is to try to brute-force FTP logins. With an exploit available, you just need to fetch a bunch of web pages looking for a particular string or two of text; you don't much care which site(s) have the exploit as long as you can get your malicious code out there somehow, to as many places as you can. A script kiddie can hit a few thousand sites in a day with a bot they found online. With the FTP thing, it means throwing some resources at a particular site/server; if you happen to stumble across an easy login, great, but unless you have a particular site you want to deface it's usually more trouble than it's worth to take that approach. Changing your password might be a good idea, but it's far less likely that FTP was their way in.