According to reports, twitter is experimenting with ads on profile pages.
I haven't yet seen them, but as long as it doesn't make it ugly, or put inappropriate ads on there, i'm ok with it. I assume the ads would be targeted towards the users profile, but that could be a worry as a user may not realise that the ad is generated as a result of the users' surfing habits, and not that of the profile being viewed.

“We’re experimenting with this feature, as we do with all our ad features, in order to create great experiences for our users, advertisers and partners,” the spokesperson said in a statement to Re/code. Twitter Experiments With Ads On Profiles [recode.net]