You can use TweetSharp v2.3.1. It supports Twitter API 1.1, and also provides full support for the .NET Framework 3.5 (as well as 2.0).
You can get it through NuGet or GitHub : if you use the NuGet package manager, you can simply install it from the command window by doing this:
Install-Package TweetSharp
I personally can guarantee that it works because I used it in an old 3.5 project and I managed to get it to work.
IMPORTANT NOTE : if your project does not include Newtonsoft Json.NET> = 5.0.6 (which is a necessary dependency), NuGet will automatically try to get an updated version that can give you .NET 3.5. If you run into this problem, simply download Json.NET v5.0.6 manually , delete all the materials downloaded by NuGet (including the .refresh file) and replace them with the Json.NET DLLs that you downloaded (in the / Net35 / folder).
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