Both sites will most likely require users to change their passwords. I recommend being proactive and changing it ASAP. Also change any other passwords that are associated with the email address used for the Monster sites; especially if the account uses the same password.
This event heightens the need for different passwords for different accounts. I know, I know . . . it’s too hard to remember all those passwords, much less matching the right password for the right account. Here’s a tip if you are having trouble: Use phrases. For a site like Monster, you can use "1nAnjp$09, for I need a new job pretty soon, 09 (for 2009)". That’s a good mix of upper/lower case letters, numbers, and special symbols. Of course, you have to work within the requirements of the website/software.
Another thing to keep an eye out is phished email. Stolen or harvested email addresses are sure to become targets for bogus attacks. As a reminder, it is not a good practice to respond to a request to click on a link in an email; especially when there’s a question of account security. Instead, use the shortcut that you most likely already saved on your desktop or in your browser favorites to get to the sites. These emails are sure to be very convincing.