Also almost all of their trading volume comes from Ripple Gateways (which are not ripple agnostic and I think they can also trade without any transaction cost, e.g. Ripple can let them just trade with themselves).
If one looks at legitimate third party exchanges (e.g. Cryptsy, Poloniex, BTC38, Kraken etc.) Ripple has combined trading volume of less than $8,000.
http://coinmarketcap.com/currencies/ripple/#marketsThe third thing is that Ripple got many millions in venture capital funding and I think some this also goes into price stabilization.