Using the masu verbs with my friends

I met a lot of Jp penpals on this website. With some of them, I kept in touch. With some (many) others, contact was stopped.

I'm a 21 y.o man and my penpals are between 20 and 30 y.o, men and women.
When I write to them, I always use the polite form, the "masu" form (example: tabemasu). I think that it doesn't annoy my penpals at all, because no one ever told me "use the "-ru" form".. However a lot of my penpals, when answering to my mails or talking with me on Skype, use the "-ru" form.

For this reason I'm always wondering: would it be better if I use the "ru" form?
Until now I always used the "masu" form because it is the most polite, and, as I've heard that Japanese politeness is very important, I am always careful about that.

Should I use the "ru" form?
Thanks!

desu, masu form is polite and grammatically correct.

ru form(eg, taberu) is informal, friend's talking way. It's okay between friends, or very well know people but not recommend to use between unknown or not well much people.

I guess as you speak someone much higher position you wouldn't speak just as your friend or family. You wold probably choose something more polite and appropriate word. It's same as that.

Are you a lefty?
Once you get to know each other as your friends, learn to use less polite/formal language. Otherwise, you/they feel there is an invisible wall & not opening up yourself. But it depends on how you feel about your pen pals.

Follow their lead.

When first contacting a new penpal, use the "-masu" form to be polite. If/when they change to the "-ru" form, you should change also. Otherwise you will start to come off as distant and a little too formal.

Obviously, I am only talking about penpals/friends, not a relationship where there might be a difference in level (superior/subordinate, etc.)