I've said all along that the best hope for more episodes after the initial 13 was a digital service like Amazon Prime or Hulu. I don't see that as particularly likely, but it's at least a possibility. Anyone still holding out hope that NBC will renew the series is kidding themselves, and the show is likely too expensive for a cable network. There are already plenty of other DC/Vertigo series on the way to cable channels (Krypton, Preacher, DMZ, Scalped, Titans, etc.) and they don't have the baggage attached that Constantine does at the moment.
As for the ratings, aside from a single 1.1 performance (the episode which followed the fantastic episode 4) every episode after the pilot has been a 0.8, 0.9, or 1.0 and it hasn't been consistent. Up one week, down another, back up again, back down. It has never risen two consecutive weeks:
http://tvseriesfinale.com/tv-show/constantine-season-one-ratings-34484/
I dare say we could be looking at a new low next Friday night, given the midseason premiere (and part two of a cliffhanger) tied the series low.
Those just aren't the kinds of numbers, even for a show on a Friday night, that would entice a network to save it. They keep claiming the DVR/digital viewership is good, and if so, that's what companies like Amazon, Netflix, Hulu, Yahoo, etc. would be looking at. Those numbers aren't going to matter to cable channels airing the show in first run.
I did enjoy last night's episode, though. I was thinking it'd be Nergal that came to save John but given the midseason finale I should have known it'd be Pazusu. Zed's plot seems to finally be going somewhere and her acting has been improving bit by bit.