Word for "butting in on the Net"

Solution 1:

Deliberately trying to irk people on the Net and trying to create controversy goes under the name of trolling.

To make a conscious attempt to provoke controversy or disagreement on the Internet (Chambers)

Any person who passes such uncalled-for remarks that are not in line with the discussion are called trolls. This Wiki link provides all details about trolling.

Solution 2:

The most obvious verb for such activity is to kibitz ("To chat; to gossip; to make small talk or idle chatter" or "To give unsolicited or unwanted advice or make unhelpful or idle comments, especially to someone playing a game"). Also consider to snark ("To express oneself in a snarky fashion", ie to make snide remarks or offer sarcasm), and aforementioned troll. I've also seen such comments referred to as drive-bys (things "done quickly or in a cursory manner") and as flame bait ("(Internet) deliberately provocative posting intended to start a flame war") or arson ("The crime of setting a fire with intent to cause damage").