(I am neither a Facebook employee nor can I speak for them.)

I really think that was just coincidence. Considering how many people talk about startups over Facebook messages and how many people have been contacted by Facebook recruiters, there's bound to be at least one person who was contacted by a Facebook recruiter a day or two after talking about startups. It'll seem mighty suspicious to that person, and they'll blog about it. Congratulations, you're the lucky one.

Companies that big just generally don't do things like read private e-mail, because they know it'll get out somehow - disgruntled employee, whistleblower - and the damage to their reputation is totally not worth whatever they can gain from it.