Japanese is often encoded in Shift_JIS, which is much better than UTF-8 (for japanese text). Most browsers default to that encoding on japanese OSes, so there probably isn't a real bandwidth issue, depending on the ratio of browser to dedicated client usage.

As for ideograms, your statement is not correct. Kanjis allow for better content/character ratios, certainly, but one kanji is very often not one word - the word foreigner, for example, uses 3 ideograms.

On top of that, Japanese is not written solely with kanjis (as opposed to Chinese, for example). It also uses katakanas and hiraganas, which stand for phonems. This is more often the case on social networks where a lot of western words are used - western words are almost systematically written with katakanas.

Kanas are still more "efficient" than alphabets, but to reuse my previous example, foreigner is written using 5 kanas instead of 3 kanjis.