PMs: Email.
Posts: Blog. [XML-RPC if you wanna email from your phone to your blog. Different post-type or tag for 'like' vs 'post' (heck, invent any 'like' 'like' you like).]
Follow: RSS.
It's all there and decentralised for years, just not in a polished package of a 140 character comment, and likely requires more than 15 seconds of thinking. Actually really straightforward.
Only tricky thing would be SMS but that's... 'depreciated' as a popular feature today.