Trade Me may have foiled its ambitions in New Zealand, but online auction site eBay has become one of the heavyweights of the internet, right up there with Amazon, Yahoo and Google.

But after a decade of growing at high double-and triple-digit rates as it expanded rapidly into new services and new countries, eBay's growth has been slowing, and some investors are concerned the party might soon be coming to an end.

The company's share price, which has dropped by more than 40 per cent over the past six months, took another slide last week after a leading brokerage firm cut its rating on the stock to "underperform".

Safa Rashtchy, of Piper Jaffray, also dropped his target price to $25 from $30.

He said recent trends at the online auction company suggest "a worsening picture for eBay".

The analyst said he was concerned that buyers were not coming to the auction site the way they once did.

This meant eBay might have to boost its advertising and marketing costs, which would cut into its profit.

The company has tried several strategies in an attempt to stimulate growth in its core auction listings, where growth has been slowing the fastest, but analysts are sceptical about their chances of success.

Growth continues to be relatively strong for eBay stores - professional or semi-professional sellers who set up their own virtual storefronts - but the company doesn't get as much money from them as it does from regular listings.

In any case, Rashtchy says, it's not sellers who are the real growth problem but a lack of buyers.

For some observers, a sign of eBay's desire to boost its growth was the purchase of Skype, the free - and unprofitable - internet phone network it this year agreed to buy for between US$2.4 billion and US$4.1 billion, depending on the company's financial performance.

Although eBay has been integrating Skype's voice-over-internet calling into some of its services, there has been little sign that it will get enough benefit from Skype to justify the purchase price.

This has increased concerns about the company's financial future.

To make matters worse, a recent report from one brokerage firm said Skype's growth also seemed to be slowing.

RBC Capital Markets analyst Jordan Rohan said increases in two main measures of growth at the voice-over-internet company - number of downloads and number of new users - had slowed since eBay bought the company.