A company’s “market share” is an important concept in business.
Market share is basically the % of the market that your company has. Coke and Pepsi both have a large market share in the soft drink industry. They are always looking for ways to increase their market share. Market share is a very important number for certain large companies. Many companies literally pay millions of dollars a year to market research companies to check on their market share and help them find ways to increase it. A few percentage points in market share is often huge.
It is very important for Facebook to have a large market share. This is because many people use Facebook just because their other friends are already using it. A website similar to Facebook would have a difficult time competing because Facebook already has such a large market share. A website like Facebook is only useful if you know that other people are using it. This is the same for job search websites. If few people are using them, it is difficult to convince new people to join.