The nature of cults is described in scripture in terms of cult leaders, but without the use of the word cult. Cults are the creation of false teachers and false prophets that seek personal gain and power over others.
2 But there were also false prophets among the people, even as there will be false teachers among you, who will secretly bring in destructive heresies, even denying the Lord who bought them, and bring on themselves swift destruction. 2 And many will follow their destructive ways, because of whom the way of truth will be blasphemed. 3 By covetousness they will exploit you with deceptive words; for a long time their judgment has not been idle, and their destruction does not slumber. 2 Peter 2:1-4
6 For of this sort are those who creep into households and make captives of gullible women loaded down with sins, led away by various lusts, 7 always learning and never able to come to the knowledge of the truth. 2 Timothy 3:6-7
17 These are wells without water, clouds carried by a tempest, for whom is reserved the blackness of darkness forever.
18 For when they speak great swelling words of emptiness, they allure through the lusts of the flesh, through lewdness, the ones who have actually escaped from those who live in error. 19 While they promise them liberty, they themselves are slaves of corruption; for by whom a person is overcome, by him also he is brought into bondage. 2 Peter 2:17-19
Cult leaders are always false teachers and always use their authority for unrighteous gain, either the accumulation of wealth and power, and typically for taking sexual advantage of cult members as well. David Koresh and Jim Jones are two modern and well documented cult leaders who lead their followers to destruction and meet those biblical descriptions of false teachers, but cults are far more common than those two would suggest, and when they survive the original cult leaders with a group of accepted elders, those elders often continue in the same error for the same unrighteous advantages over others. Eg: the practice of polygamy with the concurrent domination of young women, sometimes forced into marriage against their will.
While an online site could be cultic in its teachings, I don't see how a cult leader would profit from online ministry unless selling products or services as "prophets for hire".
A defining characteristic of cults is the threat of damnation through excommunication (the reason some evangelicals regard larger institutionalized churches as cultic), and the notable absence of an authentic gospel.