Why is Ubuntu using so much memory?

I have just turned on my PC and have two applications running; Thunderbird and Firefox. Whatever else is running is part of Ubuntu's feature set (as well as a few indicators).

This has been ongoing for weeks, so I'm not sure if it's normal behaviour or not.

Right now, my memory usage indicator says that Ubuntu is using 41% of my 6GB of RAM. I haven't even opened Gimp or my other day-to-day things. I'll be buying another 6GB of RAM, this week, so that I don't run out when using Gimp or Openshot, for example.

An overview of my running applications shows that Nautilus is using the most, yet it isn't even open. This seems a little excessive, or am I missing something?

Try running the command free -m in a terminal, and look at the second line. It shows memory usage minus memory used for caching. For instance, my basement server has the following memory usage:

                   total       used       free     shared    buffers     cached
 Mem:               2003       1663       340          0        601        462
 -/+ buffers/cache:            599       1404
 Swap:             9536         21       9515

My server uses 1663Mb memory, but only 599Mb is allocated by processes. The rest of the memory is used by the kernel as cache, so 1404Mb is free to use. Remember that totally unused memory is wasted memory. If memory is not used by processes, it should be used to cache disc etc. For instance, if you close Firefox, the program could be kept in memory so it starts faster the next time you need it. If the memory is needed by an application, it will be allocated without the need to swap it to disk.

Another thing that your screenshot doesn't tell you is how much memory is used by shared libraries, code shared between different processes of the same program etc. For instance, the different gwibber-service processes probably share much of the memory. If you want the whole picture, you need to look at virtual memory, resident memory, shared memory etc (available in the preferences).

As long as you don't feel the sluggishness of the machine swapping out memory to disk, you don't need to worry at all.