Like farming is where they make an attractive page, such as the many 'free' giveaways (unsealed Ipads, testers for new Xbox/Playstation) and then they sell the page to another person who then changes all the details and Voilà...a ready made new pages with Thousands of likes to put them higher up the Facebook search rankings.
Worse case scenario is that you give details to receive it and you get scam/junk mail or worse, fraud.
I'm pretty sure this is a scam although lots in my newsfeed seem to have fallen for it too. It's not from Tesco, but by "liking" it and filling in your details the scammers then have the means to bombard you with junk.
Lots of my FB friends seem to be doing it. Obviously, £500 is quite a nice lot of money to get, but I'm wondering a) is it genuine b) do you end up having 'Liked' Tesco so that you get them in your news feed all the time - can you 'unlike' them afterwards to stop this? c) if you get the voucher do you have to spend it all in one go or can you use it over several lots of shopping, can it be used on Tesco Direct etc as well? Thanks!