Yes, you are right, it is plural.
About the other, "mi piace un sacco" and "mi sto divertendo molto" are equivalent: maybe the first is more colloquial...
I agree with 3mend0 about mi sto divertendo molto. I would keep it, since there isn't much difference in meaning and since this request is "meaning only".
3mend0, adesso puoi correggere il testo, ho annullato la votazione. Appena lo avrai modificato lo accetterò