should I return???

Well, without any logs or error information, I'd say I don't think so... it's very possible that this episode had to do with a kernel problem related to the facebook site (and/or) the access to your external hard drive. In every mac I've got, I've gotten such episodes every once in a while (once every two months maybe?). So don't panic: Mac OS X is good (better than Windows or Linux, IMO), but it is not perfect by any means.

Nevertheless, here are some tips to deal with such episodes:

1) First, to put your mind at ease and rule out the lemon theory: pop in your install disk #2 (that came with your computer), and hold the D key while restarting to run the Apple Hardware Test. That shoud give you some info on whether something's wrong with your machine hardwarewise.

2) Repair permissions using your install disk #1. Pop in the OS X install disk, select Disk Utility, veryfy and repair permissions. Also try resetting your PRAM if this happens often.

3) Download Onyx and clean caches and rebuild your directories... more info about this procedure can be found in the Onyx help.

4) If that ever happens again with Safari or any other application or hard drive (I won't lie to you by implying it won't ever again happen)... try using the "Force Quit" feature, instead of hard rebooting the machine.

5) Remember you have 1 year warranty and 3 months of phone support (in case you didn't get Apple Care), so even if the worst case scenario comes true - you getting a lemon and none of these tips help you - my experience is that Apple will take care of it and fix it. If the problem persists, try giving them a call and take it from there.

Hope this helped. Cheers!