Social media sells.
From what to wear to exotic holidays, Instagram and Facebook influencers have become a big chunk of the advertising industry.
Brands pay celebrities to post pictures to their thousands, sometimes millions, of followers. But do we always know when something is an advert?
Social media users in London said they understand the rules, and they know when someone is advertising.
Sachelle, 18, told Sky News: "Social media users are savvy, we know that celebrities are making money on their Instagram.
"At the end of the day if I like something I'll buy it, if I don't I won't."
But others were less clear: one man said he had deleted Instagram because he did not think it was transparent and did not know which posts to trust.
Celebs need to tell followers if they are being paid to post and most do that by writing #ad under a post, or by using the "paid partnership" feature.
The Advertising Standards Authority said this was relatively new ground for them, but that they had warned between 200 and 300 social media influencers against breaching the rules.
Recent interventions by the ASA include reality TV star Stephanie Davis, who shared a photo of some vitamins she was promoting.
The photo has since been edited to include the caption #ad.
Made in Chelsea's Millie Mackintosh was also warned after uploading a video advertising a health drink using #sp to refer to a "sponsored post", which the ASA ruled was not clear enough.
Love Island 2018 contestant Zara McDermott is among the growing number of social media influencers.
She has one million followers on Instagram and says her posts provide her main source of income.
But her concern is that not everyone follows the same rules, and she accused the ASA of being too strict.
She said: "I could wear something and it will sell out on a website instantly. I always write #ad under my posts, or find a way to make it very clear because I think people want to see transparency. I'd rather be transparent than not.
"I think if you're advertising something that you're getting a monetary benefit from, the rules should be that you hashtag add or you put it as a paid partnership.
"I don't think people should be removed from social media completely but I do think they should remove the post that is causing the offence.
"Sometimes people think my images are ads and they're not - actually it's not an ad I'm just wearing some clothes.
"I think sometimes there is a bit of a label that we're a walking advertisement when we're actually not."
Andy Nairn, partner at advertising firm Lucky Generals, told Sky News that advertising regulation often works through 'carrot and stick' and so far the ASA has taken perhaps more of a 'carrot' approach.
"They have understood that there has been some degree of complexity, a novelty in the world of influencer marketing, it's a relatively new discipline.
"It's not entirely new now, so perhaps it's a little bit late in the day, but they have chosen to clarify the rules first and foremost so that people don't have the excuse of not knowing what the proper form of behaviour is."