I can't speak for Parse, but I use both of these technologies in one site. Memcached is, well, for any kind data that you want to cache: results of SQL queries, blocks of the site (HTML), etc. Redis, on the other hand, it's a final data storage on its own (like MySQL, because it's also persistent), but it's faster than MySQL for certain things. For example, for things like "List of profiles I've visited / have visited me recently on Facebook". This seems like a very simple MySQL table and query, but once it becomes big it starts to get slow. On the other hand, you can have one key per user on Redis, like "<id_user>_visit_history" which is a sorted set of <time, id_user>. You could do the same with Memcached (although it doesn't support the same data types), but then it wouldn't be persistent.
Right ... thats my point. redis is as fast if not faster than memcached for caching exactly the same things you talked about (results of sql queries, blocks of the site etc) so why not just use redis for everything and eliminate memcached.
