Plusnet make referrals payments.
Yes, I believe it is 25p per referral per month. Hardly going to make someone a fortune - I doubt if many people have much in the way of referrals at all.
From the evidence I have seen to date there are a some people that have performance issues. There were definitely problems with support for months when they were setting up their new call centre - haven't seen so many complaints about it just lately.
However, the number of people that have no particular issues (or none that they know of) and therefore score them 10/10 is likely to outweigh the number of people who have noticed issues.
I am with BT. If I rate them then I tend to rate them top as I haven't had any significant issues with the connection. Why would I rate them down if it is working fine for me? Same likely goes for PlusNet customers on here - I don't buy the conspiracy theory that it is bias or referrals - it is just the plain fact that most people have no issue and therefore no reason to score them down.