Which DOJ official and/or senator is going to go after facebook and reddit? Unless they're openly promoting sex trafficking I don't think the giants are going to have any issues.
Just to be safe, though, reddit did take down any subreddit that discussed prostitution.
The situation is bad enough that Craigslist took down their dating section. And really, any dating site or service (classics like match.com and modern services like Tinder) are easy prey for this law. I saw a headline towards the top of HN that Facebook is dabbling in online dating, which puts the biggest social media company in history in the crosshairs.
In practice, all someone would have to do is post a few underage images to /r/gonewild or similar high-traffic subreddits. Or, for that matter, simply drop random links in unrelated subreddits.
SESTA is a weapon, not a defense. A metaphorical loose cannon.
