Complex sentence: Online recruitment websites can be very helpful to find candidates that are very actively looking for work and post their resumes online, but they will not attract the "passive" candidates who might respond favorably to an opportunity that is presented to them through other means.
Split into simpler sentences: Online recruitment websites can be very helpful to find candidates that are very actively looking for work and post their resumes online. Online sites may not attract the "passive" candidates who might respond favorably to an opportunity presented to them through other means.