Was this also not true of Standard Oil and AT&T?

I do not believe they need to prove a company is being anti-competitive. If that's the case, I would assume the majority of cost would be legwork in gathering evidence.

Article is paywalled for me, but wouldn't the relevant comparison be to Facebook's legal and lobbying budget?

If we're comparing overall organizational resources, we should compare FB's revenue to the entire DoJ or US Government.

This generic argument against taxes and regulation doesn't apply to an antitrust discussion.

The point is a giant company can easily outspend the government and bury it. This runs counter to your "think of the little guy!!!" argument being used to defend Facebook.