AKA Argument from popularity. Appealing to a group's beliefs or values as an authority. The fallacy occurs in assuming a view is correct on the basis of how many people believe it. The popularity of an idea cannot tell us wether or not it is true. To find out if a proposition is true, you must examine the arguments and evidence rather than counting the number of people who believe it.
1."The vast majority of people on earth believe God exists, therefore God exists"
2."Most my friends on Facebook are posting this article, therefore it must be true ."