cracking is the most inefficient way to get into someone's facebook or myspace.
basically, cracking is just having a program go through a list of passwords until it gets it right.
if you know their login email address, you can use a cracking program (with rotating proxy) to try and get in, but it will take forever.
The better ways have already been mentioned but here is the correct order:
1) backdoor exploit (not likely to find one on either of those sites)
2) "social engineering". basically getting them to tell you their password out right.
3) A keylogger on a machine they use to login (THIS IS PROBABLY THE EASIEST IF YOU SPEND ANY TIME WITH THE PERSON)
4) a phishing website. (terribly difficult to use now a days, with all the anti-phishing going around. if you hack their hosts file so myspace.com points to your site, it would work best. but if you're able to hack their hosts file, you could just use a keylogger)
5) One that people rarely think of: check their firefox. go to preferences/security/show passwords then click the show passwords button. It would work on me, and I'm a security prone person!