"Many web sites also feature a network effect. One example is web marketplaces and exchanges, in that the value of the marketplace to a new user is proportional to the number of other users in the market. For example, eBay would not be a particularly useful site if auctions were not competitive. However, as the number of users grows on eBay, auctions grow more competitive, pushing up the prices of bids on items. This makes it more worthwhile to sell on eBay and brings more sellers onto eBay, which drives prices down again as this increases supply, while bringing more people onto eBay because there are more things being sold that people want. Essentially, as the number of users of eBay grows, prices fall and supply increases, and more and more people find the site to be useful." (Bold is mine)
As the bold section states "eBay would not be a particularly useful site if auctions were not competitive." and this is exactly the dynamic we are seeing on eBay today. There are fewer and fewer reasons to participate in the eBay marketplace today and the Network Effect works in both directions.
I contend that the current strategy of eBay management will hasten the reversal of the Network Effect and that hissing sound (slow leak) you hear today will turn into a complete blow-out within 12 months.
Just my 12%