An average person has a strong social relationship with about 150 people, realistically varying from 100 - 230 people.

A strong relationship is defined by:

Everyone living in a close proximity to each other
Everyone knowing personal information about individuals in the group
Everyone understanding how the group feels about each other

Social media allows people to expand their social group, but the relationship between users is considered a "weak tie" since you can interact with strangers or acquaintances that you may not live near or know that well