I've been flying weekly between DFW and SFO. At DFW (admittedly on Fridays) I would say every 3rd or 4th person gets the triple beep - mostly Delta people I think. At SFO I've never seen anyone else using pre-check. I'm surprised that VX doesn't have more of a campaign to encourage people to sign up for Global Entry.
On another note, please please open pre-check at LAX T3. What a mess.