Rabbi Ovadiah Yosef was asked if one needs to ask forgiveness from others in person, or if it could be done via messenger or letter (and I would update that to include email and Facebook posts).
Rav Yosef begins his reply by discussing the general severity of the requirement to ask forgiveness from others before Yom Kippur. There is a debate in the Gemara and through the Rishonim and Acharonim whether repenting before God and repenting before our fellow man are two separate requirements that can exist separately, or whether failure to do one holds back our performance of the other. In other words, if one fails to gain forgiveness from others, there is a view that his sins before God are not forgiven either. The general reason given for this stricter approach is that if one does not seek forgiveness from his friend, then he is rejecting that mitzva of the Torah, and thus his repentance before God loses some of its meaning.
[Mori V'Rabi Rav Aharon Lichtenstein noted in a teshuva lecture in 1997 that this idea is based on the last mishna in Yoma, where Rabi Akiva notes that we are fortunate to become purified before Hashem, based on the verse מקוה ישראל ה' - Rav Lichtenstein pointed out that there is something frightening in this statement. Since one who enters a mikveh holding a שרץ is not purified, so too one who repents before God while still holding onto his sins vis-a-vis others also cannot become fully purified and forgiven.]
After concluding his lengthy introduction about the necessity of seeking out others to ask them for forgiveness, Rav Yosef rules that it is preferable to ask someone for forgiveness in person. While it is true that Yosef's brothers sent a conciliatory mission to him after Yaakov's death and Aharon HaKohein would serve as a go-between when making peace between adversaries, nevertheless the preferred approach is that one confront his friend personally and ask for forgiveness. In certain situations, depending on either the stature of the individuals involved and/or the severity of the offense, it may be prudent to first send someone else to lay the groundwork for the eventual rapprochement.
Rav Yosef concludes by noting that it is important for children to ask forgiveness from their parents, as people constantly stumble in their performance of the mitzva to respect their parents. Similarly, husbands and wives must ask forgiveness from one another, and students must ask forgiveness from their teachers.
Wishing everyone a גמר חתימה טובה.