I've had the opposite experience with Last.fm. I actually realized last week that my scrobbling was broken, and I wasn't particularly disappointed.
Last.fm used to be a place where I could check out bands I had never heard of. It was like the website equivalent of the friend who spends all day listening to new music, and will recommend music that is eerily perfect for you.
I feel like the "similar artists" function has changed to prioritize artists that are already extremely popular, even if a much less popular artists is much more similar to the band you are currently listening to. It's plausible I know a lot more bands than I used to, and that new bands are inherently less novel to me. However, I believe this is a trend that can be followed across almost all websites.
As evidence, you only need to look at the changes Facebook made in the same time period. They study the interactions their users make as much as any company, and they've ensured that their users spend the most time on Facebook and provide them with as much data as possible. Facebook used to show every update on the new feed. Now I rarely see posts from anyone I disagree with. I used to get a lot of my information about bands from their Facebook groups. Now the only updates I get are from the major companies I've liked, because they can pay to send a message.
I totally understand why large websites are doing this. These services are most profitable as an advertising platform. In the case of Last.fm, the people listening to John Doe and the Backup Band aren't going to make them money. This 'cultivation' has an extremely chilling effect on the variety and depth of news, viewpoints, etc. that allowed the internet to foster an explosion in creative expression.