Their main value proposal is that they aren’t backed by Facebook, but other than that, there’s no real difference to end user.
That’s enough for geeks, but not very compelling argument to break to mainstream. To break into such a highly competitive market, they need more unique features.
Or they can count on Facebook to keep on shooting themselves into foot, which is possible, but historically risky bet (no matter what HN crowd says, they continue to be extremely successful and know how to grow products).