Some Questions from a newbie

Hello highscalability world. I just discovered this site yesterday in a search for a scalability resource and was very pleased to find such useful information. I have some questions regarding distributed caching that I was hoping the scalability intelligentsia trafficking this forum could answer. I apologize for my lack of technical knowledge; I'm hoping this site will increase said knowledge! Feel free to answer all or as much as you want. Thank you in advance for your responses and thank you for a great resource!

1.) What are the standard benchmarks used to measure the performance of memcached or mySQL/memcached working together (from web 2.0 companies etc)?

2.) The little research I've conducted on this site suggests that most web 2.0 companies use a combination of mySQL and a hacked memcached (and potentially sharding). Does anyone know if any of these companies use an enterprise vendor for their distributed caching layer? (At this point in time I've only heard of Jive software using Coherence).

3.) In terms of a web 2.0 oriented startup, what are the database/distributed caching requirements typically needed to get off the ground and grow at a fairly rapid pace?

4.) Given the major players in the web 2.0 industry (facebook, twitter, myspace, PoF, Flickr etc, I'm ignoring google/amazon here because they have a proprietary caching layer) what is the most common, scalable back-end setup (mySQL/memcached/sharding etc)? What are its limitations/problems? What features does said setup lack that it really needs?

Thank you so much for your insight!