The Bitcoin price is determined by demand and supply.  If people want to invest in a HYIP they need Bitcoins, so demand goes up.  This results in a price increase.

Back in December 2011 the Bitcoin price was at a local minimum around $2.  BTCST was started at about the same time, and we had a constant upward trend since then.  This was not only due to BTCST (e.g. SatoshiDICE was lunched in April), but BTCST contributed to the upward trend.

I also discuss this in my medium-term market analysis.