Facebook is not a monopoly. There are other social media firms out there, it did not achieve its market share primarily through anti-competitive conduct, and there are almost no formal barriers to entry into the social media space.
It is nonetheless possible that Facebook may have engaged in anti-trust violations (not necessarily monopolization or attempted monopolization), which do not require that an anti-trust offender have a monopoly.
Facebook has a very large share of the relevant market share (although exactly what market is relevant is a key and hotly contested question of fact in anti-trust litigation). And, this gives it the economic and market power to engage in anti-competitive conduct if it chooses to do so. But there is little evidence that anti-competitive conduct is what caused it secure or maintain its large market share.
The primary U.S. anti-trust law regulating monopolies and attempts to form monopolies is Section 2 of the Sherman Act. 15 U.S.C. § 2.
Section 2 of the Sherman Act makes it unlawful for any person to
"monopolize, or attempt to monopolize, or combine or conspire with any
other person or persons, to monopolize any part of the trade or
commerce among the several States, or with foreign nations . . . .
For purposes of Section 2 of the Sherman Act, as explained by the U.S. Department of Justice at the link above:
Monopolization requires (1) monopoly power and (2) the willful
acquisition or maintenance of that power as distinguished from growth
or development as a consequence of a superior product, business
acumen, or historic accident.
and
Attempted monopolization requires (1) anticompetitive conduct, (2) a
specific intent to monopolize, and (3) a dangerous probability of
achieving monopoly power.
Furthermore:
Today, a consensus--as reflected in both judicial decisions(33) and
the views of a broad cross-section of commentators--exists on at least
seven core principles regarding section 2, each of which is discussed
in the sections that follow:
- Unilateral conduct is outside the purview of section 2 unless the actor possesses monopoly power or is likely to achieve it.
- The mere possession or exercise of monopoly power is not an offense; the law addresses only the anticompetitive acquisition or maintenance
of such power (and certain related attempts).
- Acquiring or maintaining monopoly power through assaults on the competitive process harms consumers and is to be condemned.
- Mere harm to competitors--without harm to the competitive process--does not violate section 2.
- Competitive and exclusionary conduct can look alike--indeed, the same conduct can have both beneficial and exclusionary effects--making
it hard to distinguish conduct that should be deemed unlawful from
conduct that should not.
- Because competitive and exclusionary conduct often look alike, courts and enforcers need to be concerned with both underdeterrence
and overdeterrence.
- Standards for applying section 2 should take into account the costs, including error and administrative costs, associated with courts and
enforcers applying those standards in individual cases and businesses
applying them in their own day-to-day decision making.
The Federal Trade Commission, which is the primary federal government regulatory agency charged with enforcing U.S. anti-trust laws further explains the nuances of what is and is not an anti-trust violation. It notes that:
Section 2 of the Sherman Act makes it unlawful for a company to
"monopolize, or attempt to monopolize," trade or commerce. As that law
has been interpreted, it is not illegal for a company to have a
monopoly, to charge "high prices," or to try to achieve a monopoly
position by what might be viewed by some as particularly aggressive
methods. The law is violated only if the company tries to maintain or
acquire a monopoly through unreasonable methods. For the courts, a key
factor in determining what is unreasonable is whether the practice has
a legitimate business justification.
It goes on to explain that conduct which is relevant includes: (1) exclusive supply or purchase agreements, (2) predatory or below-cost pricing, (3) refusal to deal, and (4) tying the sale of two products.
Admittedly, this only scratches the surface of what does and does not constitute a monopoly related anti-trust violation. But, the academic literature and case law addressing these issues is vast, despite the fact that anti-trust litigation is very rare and rarely makes much of a big picture difference in the economy.
