Linear regression.
[Facebook] RMSE: 664.1011224268102
[Facebook] R^2: 0.013521214179805008
[GooglePlus] RMSE: 20.39986222623975
[GooglePlus] R^2: 0.005860464236229657
[LinkedIn] RMSE: 98.31866519340444
[LinkedIn] R^2: 0.001350236016830486

Polynomial regression, degree = 2.
[Facebook] RMSE: 662.4072650867852
[Facebook] R^2: 0.01854702517889739
[GooglePlus] RMSE: 20.336481305037495
[GooglePlus] R^2: 0.012028309479154897
[LinkedIn] RMSE: 98.24156233242569
[LinkedIn] R^2: 0.0029159318492958386

Polynomial regression, degree = 3.
[Facebook] RMSE: 656.7965896385655
[Facebook] R^2: 0.03510268068137912
[GooglePlus] RMSE: 20.32247017285454
[GooglePlus] R^2: 0.013389197157252175
[LinkedIn] RMSE: 98.15982241858943
[LinkedIn] R^2: 0.004574449039282835

Polynomial regression, degree = 4.
[Facebook] RMSE: 657.0556747659788
[Facebook] R^2: 0.034341288629486044
[GooglePlus] RMSE: 20.32892537590358
[GooglePlus] R^2: 0.012762326064028384
[LinkedIn] RMSE: 98.30736158440001
[LinkedIn] R^2: 0.0015798505571963384