Can FTRL be applied on linear least squares? or is it just for logistic regression models? I'm exploring follow-the-regularized-leader FTRL proximal gradient descent:
paper, reference implementation.

Everywhere FTRL is mentioned, the loss surface for the gradient decent is the LogLoss, and the model for prediction is Logistic regression. 

Can I use the same algorithm for a linear least squares model? I have a problem I want to model with a linear model and define the loss by least squares, and then do FTRL to find the optimal solution - do you see any problem with that?

Thanks.
As we can see, FTRL is nothing to do with our model, and it only serves as a way of approximating the optimal solution , just as SGD does.

Logloss is just a kind of loss pattern in classification problem,and there are other alternatives for it , such as ExpLoss. Least-squres loss is one of all loss patterns in regression problem.

The reason why FTRL was mentioned everywhere ,which mainly treated Logloss as a example, is that it was often used in classification problem , such as CTR model in online advertisement scenario.

Hopes this can helps you ,and good luck !