The main metrics to look at is:
- User count and growth: How many users do you have, and how is this growing
- Retention: How many users come back to your site
- Activity: This is define by whatever you provide, but basically you want to know if users are actually doing something on your site
Assuming you sell something:
- Conversion rate (very importante!!): How many users go from free to Paying
- Incremental Sales: in SaaS you have recurring revenue, so by how much does this revenue grow over a given period.
- How many of your customers would recommend you to others?
- How many of your customers have recommended you?
- How many of your customers like you?
- How many times do you loose against your competitors in a deal (or do you win?)
Other stuff to have:
- Customer Acquisition Cost: How much do you have to spend (time + Money + other) to get a customer to sign-up and to convert.
- Life Time Value (tough to calculate but critical): what's a user worth during its life time with you (expected / average)
- Incremental Cost per new User (how much does it cost you to add a new user, careful, at the beginning it might be almost 0 but as you scale there are some incremental steps like adding a server, adding an Sys Admin, etc..)
There are others for sure. To answer your question I would say that you could present those numbers based on assumptions if you are in the Business Plan/pitch phase but they will likely to be wrong. Maybe the investor just wants to see if you have a good understanding of your business and what matters..