How to optimize a log process in MySQL?
Some simple arithmetic:

100k users * 1m events per user per year * 100 bytes per event =~ 10 TB per year

In MySQL, single table is limited in size at about 4TB. I would think it should be bigger than that, but this is what official doc says.

In other words, you would be effectively forced to split or partition your huge table into set of smaller ones.

Alternatively, you can use NoSQL databases like Hadoop or MongoDB.
But, it can be painful if you are used to do everything in SQL.