I haven't done it yet, but VPNs are supposed to be a way of stopping the marketing companies getting a hold of your details, and so stops them bombarding you with adverts. Even on this site, I get adverts for things I've been looking at on eBay, so I'll be following this thread carefully.
As has been raised by others, this will mainly be driven by your browser cookies. Clearing these, or using a different browser should see these targetted ads reduce.
Free VPN's usually have some sort of advertising built into them or are time restricted (say 30 / 60 mins)
I've just gone for IPVanish, it seems to be well respected and has some good reviews, I paid $58 (USD) for 12 months it has a huge choice of servers / countries and is fairly easy to setup and use. And doesn't affect internet speed too much.
Approximately 90 pence per week for peace of mind when streaming / downloading = no brainier. By choosing a server from another country you can watch geographically restricted TV from another country (watch BBC iplayer from abroad)
What is 'WebRTC' https://www.perfect-privacy.com/webrtc-leaktest/
What is 'WebRTC' (and other stuff) divulging ? See 'Demo' https://github.com/diafygi/webrtc-ips
I suppose even if your IP number is not divulged there are plenty of things which may 'fingerprint' your actions.
Flash player needs some attention, it may make one's real IP addy available...
Just to add to the conversation you can get an infinite (100yr) deal from VPN unlimited - normally $500 for about £30 or so via Stack Social. I got this deal and it's legit - it might not be the best VPN but it does allow access to streaming etc. and £30 for 100 yrs is pretty cheap!