While Greplin is impressive, it's not in the same scale as Google, even in its early day. Google built an large global index for everyone, while Greplin built many small indices for many users. Some calculation would illustrate the point.
Google's global index: 1 billion documents. Searchable by 1 million users. Need to support 1B x 1M search capacity.
Greplin's individual indices: 1000 documents/user for each individual index. With 1 million users, there are 1B documents total. Each user only searches his 1K index. Only need to support 1K x 1M search capacity.
Google's global index: 1 billion documents. Searchable by 1 million users. Need to support 1B x 1M search capacity.
Greplin's individual indices: 1000 documents/user for each individual index. With 1 million users, there are 1B documents total. Each user only searches his 1K index. Only need to support 1K x 1M search capacity.
It's orders of magnitude difference.