According to the HelloFacebookSample from the SDK (and my own experience!):
FBShareDialogParams *p = [[FBShareDialogParams alloc] init];
p.link = [NSURL URLWithString:@"http://developers.facebook.com/ios"];
BOOL canShareFB = [FBDialogs canPresentShareDialogWithParams:p];
canShareFB will return YES if the Facebook app is installed in the system; returns NO if no Facebook app is found.