Should I keep a webpage
or blog while I am overseas?

Would that be a good way to keep in touch?

Why not! 

A webpage or a blog is a good way to communicate with family, friends, and old coworkers about where you are and what you are doing.  It's also a good way to let people know that teaching English overseas is a serious business, that you are working hard, and that it is not all fun and games (an important message for your past supervisors and coworkers especially!).

Work and Play

While the temptation might be to show off the travel and play segments of your life overseas, it might also be useful to also communicate about the cultural knowledge, languages, and other skills you will be acquiring.

Documenting your Life Overseas

One great thing about a website or blog will be the record you have made - to review later in life.  A nice documentary of times overseas if you go back home to live.  And a nice step in your development if you remain overseas.

Pre-Internet Days

My mother saved all my letters from my Peace Corps days and gave them back to me after my time in Africa was finished.  It was a great gift as I had kept no other documentation of my two years there.  It was also good proof to me of my good efforts to increase communication with my parents when overseas.

Do it!

You'll be happy that you documented your time.