how do fan pages on instagram or facebook or something get so many followers?

[Pedro]

People can either like content on your own blog or site if you have the proper plugin/apps installed) or they can like your Facebook fan page itself. Put your Facebook page's address on your business cards, prominently link to it on every page of your site, mention it in person when you get a chance, put it in your email signature for every time you send out an email, etc. Send out a post where you ask everyone to check out and like your page. If you have a lot of friends and they're good people, that'll get you a ton of likes fast. This can help you especially when you're just starting out with a new page because that initial burst of fans of your page will act as social proof and get your non friends/strangers who check out your page to like it, as well, after seeing others have done it before.