Warning: There is no such thing as free followers
All sites that offer "free followers" want something in return. Any that say they don't are probably scam sites and should be avoided. That being said, you don't always have to pay cash for your followers. The sites I am going to direct you to give you followers in return for action, for example following other people.
Warning: Avoid Robots
There are websites that offer 1000s followers. Stay away! They are likely to be fake profiles. Being followed by a lot of fake profiles can make your profile seem fake and get you banned.
The sites below offer real human followers, as far as I can tell.
Twitter Followers
TraffUp offers Twitter followers, likes and retweets in return for completing minor tasks, including visiting websites for 20 seconds, following people on Twitter, liking tweets and retweeting.
You earn points (usually 40-50) for completing each task. You can then spend those points on Twitter followers, likes and retweets. You specify your profile for followers and the tweets you want liked or retweeted, as well as how many points you are willing to 'pay' for each (minimum 50).
For example, you can earn 250 points by visiting 5 websites. You can then buy 5 Twitter followers at 50 points each.
Instagram Followers
Mr Insta offers 25 followers in return for following 10 profiles. You can use the site once every 12 hours. The followers usually arrive within minutes.
Facebook Page Likes
I have not been able to find a legitimate website that offers real Facebook page likes. I've found a few that appear legitimate, but in the final step you need to take part in several offers, most of which involve entering your credit card details in to a website.
If you have found a legitimate website, similar to the two above, for Facebook, please post a comment.
Final Warning
The followers you will get from these sites are only following you in return for getting followers themselves. They are not interested in you or your product and you are never going to be able to sell to or recruit them.
They are useful for getting started and making your profile look more real because it has genuine followers, but that's it. Don't make them part of your long term social media strategy. Just use them to help you get started.