Q. During sefira I twitter a reminder in the early night to the members of our college students group to count sefira. It has happened that I have forgotten to count myself later on when I miss mariv in shul. Would I be yotze just with the writing of that twitter on my i-phone, if I intended that in case I forget later that should count?

A. Poskim disagree if writing is tantamount to speaking in regards to counting sefira. Kol Eliahu (30), Ksav Sofer (Y.D. 106) quoting Chasam Sofer, rule that even if writing is not generally equal to talking, in regards to counting sefira it is. Conversely, Birkei Yosef (489: 14), Mahari’a Asad (Y.D. 316), maintain the opposite. Even if writing is by and large considered like verbalizing, in sefira it is not since the Torah requests counting. (See also Teshuvos Rabi Akiva Eiger 29 and 30)
Since Poskim disagree, one who counted by writing should if able, count again articulating the count without a brocho, or someone else who has not yet counted should recite the brocho for him.
However if the day has passed, and he did not repeat verbally the count, Poskim maintain that he can still count the rest of the days with a brocho. The reason being that this situation constitutes a double doubt or sfek sfeko to be lenient. That is maybe the Halacha follows the opinion that after missing one day you have not lost the mitzvah, and even if you did, maybe writing is indeed the same as saying.(Mekadesh Yisroel – Sefirah 31, Mitzvas Sefiras Haome 44)
Horav Shlomo Miller’s Shlit”a opinion is that although for many mitzvos and prohibitions the writing on a screen may be considered only temporary or even not writing at all, (See question 383 in this forum in regards to erasing Hashem’s name from a screen, see also question 465 and 466 in regards to e-readers) however, as far as sefira is concerned it is deemed to be writing and the ruling above would prevail. Therefore the person writing the twitter would be able to count the rest of the days with a brocho.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a