I think that locking your page is total bounce killer and wouldn't recommend it. However, if you are using wordpress you can social lock some of your content with onepress social locker. I also use sharelocker plugin. Although sharelocker wasn't free it costs $16. I was able to force my traffic to share on their wall and then like my fan page. Really genius stuff. Yet, in the end it cut my conversion rate in half so I unplugged the social experience from my site.