I don't think that's the best qualifier since anyone with a Facebook or Twitter account can go ahead and 'set up' a competition on their page. There are however a few things you need to check for which would generally give you a good idea about whether competitions are legitimate or not.
Who's offering the competition. Is it a well established brand? Some new companies also run competitions as part of their launch however and just because they're new, doesn't mean they're not to be trusted so don't dismiss it immediately.
Are the competition terms and conditions available on the website you're trying to enter on? If they aren't, request for them to be emailed to you by the site owners.
Are the contact details of the promoters, whether it be the company itself or their digital marketing agencies available? We see a lot of small competitions pop up on Facebook from unknown brands without any terms and conditions or contact details available. Be sure you trust the brand before entering these.
What are the entry terms? Some sites do use a referral system but when you're requested to enter 10 of your friends email addresses, be cautious.
Use common sense. Don't fall for scam SMSs and Emails informing you of your winnings in a competition you never entered. Also, do not enter a competition if it requires you to give up too much personal information.
The easiest thing to do when in doubt about the legitimacy of a competition you found online is to pick up the phone and call the company offering the competition. You will very quickly be able to establish whether or not the competition is real.
Even better, just use Win Stuff to find all your
online competitions. We do our best to make sure the competitions we list can be trusted.