There are companies that help you run contest / giveaway. In a nutshell you can do a contest where there are multiple sponsors that offer some sort of prizes. For example your 10k page can offer $100 Sephora gift card while another sponsor could be beauty brand that offers some of their product and so on. So let's say you have 10 sponsors who collectively offer 10 prizes worth $1000 and 100k total followers. They will all post contest details on their pages and the rules of the contest will be that participants must follow / like all of the sponsors + tag multiple friends + post on FB / etc. All the contest sponsors tend to be in similar niches and cross promote and therefore get each others followers andt their friends.