How can someone, halachically, do teshuva when s/he has offended an anonymous person on the internet?

Question

Ramba"m among others, state that Yom Kippur does not atone for sins committed between a person and his neighbor until a person personally asks the other for forgiveness.

Mi Yodeya is an example of meeting people anonymously. With, perhaps, a few exceptions, no one personally knows anyone behind the user ID. (I sometimes don't know myself :-)

If someone offended someone else on a web forum such as this one, or a blog post or comment, where s/he doesn't know who read his/her posts, what would be the proper or best halachich way to ask for teshuva? Can a generic "I'm sorry to whomever I offended" be used, or is there some other preferred method?

Answer 1

Chovos Halevavos - #7 - Shaar Teshuva ch.10

ch.9

But for the sins towards G-d and man, it will be difficult to repent for several reasons: He may not be able to find the person he oppressed, or the person died or moved far away...

Perhaps the oppressed will not forgive him for what he oppressed him or hurt him physically, or spoke badly of him.

The oppressor may not know whom he oppressed, or he does not know the amount of money involved. For example, if he oppressed the people of a city or a province, and he does not know them, and he does not know the amount of money he took from them wrongfully...

ch.10

...Whatever category it belongs to, if the repentance is difficult due to one of the factors we previously mentioned (last chapter) which cause repentance to be difficult, then if the person takes on himself to fulfill the requirements of repentance with all its conditions to the utmost extent that is in his power and ability to do, then the Creator will make his repentance easier, and will pardon what is hidden from him and not in his ability to do, and will give him an opportunity approximately close to his sin and allow him to absolve himself in this way (as will be explained)...

...If the oppressed is far away, the Creator will arrange their meeting, and the oppressor will submit before the oppressed and will be forgiven by him.

If he doesn't know the number of people he oppressed and the amount of money he took, the Creator will give him the opportunity to spend his money in some kind of public project, such as building a bridge, digging a well to benefit the public, or digging water pits in roads where water is scarce, or other similar things to benefit the masses, until the benefit will serve the one he oppressed and also the one he did not oppress...

...Repentance is not withheld from a sinner, rather the obstruction comes from his own wickedness and deceitful heart. But if he sincerely wants to draw near to G-d, the gate of repentance will not be closed before him, and no obstacle will prevent him from reaching it. Rather, G-d will open for him the gate of the just, and teach him the good path in His mercy and in His goodness, as written "Good and upright is the L-ord: therefore will he teach sinners in the way" (Tehilim 25:8)

Answer 2

Based on Rambam you refer to - הלכות תשובה - פרק שני

ט: אֵין הַתְּשׁוּבָה וְלֹא יוֹם הַכִּפּוּרִים מְכַפְּרִין אֶלָּא עַל עֲבֵרוֹת שֶׁבֵּין אָדָם לַמָּקוֹם כְּגוֹן מִי שֶׁאָכַל דָּבָר אָסוּר אוֹ בָּעַל בְּעִילָה אֲסוּרָה וְכַיּוֹצֵא בָּהֶן. אֲבָל עֲבֵרוֹת שֶׁבֵּין אָדָם לַחֲבֵרוֹ כְּגוֹן הַחוֹבֵל אֶת חֲבֵרוֹ אוֹ הַמְקַלֵּל חֲבֵרוֹ אוֹ גּוֹזְלוֹ וְכַיּוֹצֵא בָּהֶן אֵינוֹ נִמְחַל לוֹ לְעוֹלָם עַד שֶׁיִּתֵּן לַחֲבֵרוֹ מַה שֶּׁהוּא חַיָּב לוֹ וִירַצֵּהוּ. אַף עַל פִּי שֶׁהֶחֱזִיר לוֹ מָמוֹן שֶׁהוּא חַיָּב לוֹ צָרִיךְ לְרַצּוֹתוֹ וְלִשְׁאל מִמֶּנּוּ שֶׁיִּמְחל לוֹ. אֲפִלּוּ לֹא הִקְנִיט אֶת חֲבֵרוֹ אֶלָּא בִּדְבָרִים צָרִיךְ לְפַיְּסוֹ וְלִפְגֹּעַ בּוֹ עַד שֶׁיִּמְחל לוֹ. לֹא רָצָה חֲבֵרוֹ לִמְחל לוֹ מֵבִיא לוֹ שׁוּרָה שֶׁל שְׁלֹשָׁה בְּנֵי אָדָם מֵרֵעָיו וּפוֹגְעִין בּוֹ וּמְבַקְּשִׁין מִמֶּנּוּ. לֹא נִתְרַצָּה לָהֶן מֵבִיא לוֹ שְׁנִיָּה וּשְׁלִישִׁית. לֹא רָצָה מְנִיחוֹ וְהוֹלֵךְ לוֹ וְזֶה שֶׁלֹּא מָחַל הוּא הַחוֹטֵא. וְאִם הָיָה רַבּוֹ הוֹלֵךְ וּבָא אֲפִלּוּ אֶלֶף פְּעָמִים עַד שֶׁיִּמְחל לוֹ‏

If you know that you [might have] offended a specific user-ID, then it would make sense to apologize to that user-ID.

You would also be expected to remove the offensive writing, if at all possible.

Since MiYodea doesn't have any way to directly contact a user-ID (AFAIK), then you probably should catch their attention - try @user-ID'ing him in a thread they were recently active in. (I'm sure the Mods would prefer you do this on Meta, if possible.)

Once you know they saw the apology, you can delete it, if the offensive writing has been deleted. (If you cannot remove the offensive writing, then you probably should keep the apology visible, for future reference. People may get insulted again, forgetting they forgave you previously.)

If you suspect that something you wrote offended a group of people - and you have no idea who those would be - then it's probably sufficient to apologize with a generic "I'm sorry to whomever I offended" - somewhere close to the [edited out] offense, if possible.

Even then, if you know of some specific user-IDs who were offended, you should treat them as individuals.