Its social web services initiatives have been patchy at best, so Google hasn't managed to muscle in on Facebook or Twitter like it has with Microsoft and Yahoo.
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Patchy is to describe their initiatives as only covering some small parts of what it could be covering.
At best is to imply that the description patchy is perhaps generous.
The writer probably means to say that the quality of Google social web services has been inconsistent, tending to poor.
I doubt they are referring to coverage (another possible interpretation of patchy) since by that measure any service provider, including Facebook and Twitter, would also fall short, making the comparison pointless.
Interestingly, Etymonline has patchy as derived from the second definition of patch below.
And NOAD on the phrase at best (coincidentally using patchy as well):
It's sometimes used in parallel with at worst: